Fix thumbnail generation and add preview window

Fixed Issues:
- Exit 234 error: Added font parameter to drawtext filter for individual
  thumbnails (was missing, causing FFmpeg to fail)
- Output directory: Changed from temp to video's directory, creating a
  folder named "{video}_thumbnails" next to the source file

New Features:
- Added video preview window to thumbnail module (640x360)
- Split layout: preview on left (55%), settings on right (45%)
- Preview uses same buildVideoPane as other modules for consistency

The thumbnail module now has a proper preview window for reviewing
the loaded video before generating thumbnails, and outputs are saved
in a logical location next to the source file.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

internal/thumbnail/generator.go

@@ -234,7 +234,7 @@ func (g *Generator) generateIndividual(ctx context.Context, config Config, durat
 			seconds := int(ts) % 60
 			timeStr := fmt.Sprintf("%02d:%02d:%02d", hours, minutes, seconds)
 
-			drawTextFilter := fmt.Sprintf("scale=%d:%d,drawtext=text='%s':fontcolor=white:fontsize=20:box=1:boxcolor=black@0.5:boxborderw=5:x=(w-text_w)/2:y=h-th-10",
+			drawTextFilter := fmt.Sprintf("scale=%d:%d,drawtext=text='%s':fontcolor=white:fontsize=20:font='DejaVu Sans Mono':box=1:boxcolor=black@0.5:boxborderw=5:x=(w-text_w)/2:y=h-th-10",
 				thumbWidth, thumbHeight, timeStr)
 
 			// Replace scale filter with combined filter
@@ -363,12 +363,12 @@ func (g *Generator) buildMetadataFilter(config Config, duration float64, thumbWi
 	// Create filter that:
 	// 1. Generates contact sheet from selected frames
 	// 2. Creates a blank header area
-	// 3. Draws metadata text on header
+	// 3. Draws metadata text on header (using monospace font)
 	// 4. Stacks header on top of contact sheet
 	filter := fmt.Sprintf(
 		"%s,%s,pad=%d:%d:0:%d:black,"+
-		"drawtext=text='%s':fontcolor=white:fontsize=14:x=10:y=10,"+
-		"drawtext=text='%s':fontcolor=white:fontsize=12:x=10:y=35",
+		"drawtext=text='%s':fontcolor=white:fontsize=14:font='DejaVu Sans Mono':x=10:y=10,"+
+		"drawtext=text='%s':fontcolor=white:fontsize=12:font='DejaVu Sans Mono':x=10:y=35",
 		selectFilter,
 		tileFilter,
 		sheetWidth,

main.go

@@ -9504,12 +9504,16 @@ func buildThumbView(state *appState) fyne.CanvasObject {
 		state.thumbRows = 6 // 4x6 = 24 thumbnails
 	}
 
-	// File label
+	// File label and video preview
 	fileLabel := widget.NewLabel("No file loaded")
 	fileLabel.TextStyle = fyne.TextStyle{Bold: true}
 
+	var videoContainer fyne.CanvasObject
 	if state.thumbFile != nil {
 		fileLabel.SetText(fmt.Sprintf("File: %s", filepath.Base(state.thumbFile.Path)))
+		videoContainer = buildVideoPane(state, fyne.NewSize(640, 360), state.thumbFile, nil)
+	} else {
+		videoContainer = container.NewCenter(widget.NewLabel("No video loaded"))
 	}
 
 	// Load button
@@ -9540,26 +9544,6 @@ func buildThumbView(state *appState) fyne.CanvasObject {
 	})
 	clearBtn.Importance = widget.LowImportance
 
-	// Thumbnail count slider
-	countLabel := widget.NewLabel(fmt.Sprintf("Thumbnail Count: %d", state.thumbCount))
-	countSlider := widget.NewSlider(3, 50)
-	countSlider.Value = float64(state.thumbCount)
-	countSlider.Step = 1
-	countSlider.OnChanged = func(val float64) {
-		state.thumbCount = int(val)
-		countLabel.SetText(fmt.Sprintf("Thumbnail Count: %d", int(val)))
-	}
-
-	// Thumbnail width slider
-	widthLabel := widget.NewLabel(fmt.Sprintf("Thumbnail Width: %d px", state.thumbWidth))
-	widthSlider := widget.NewSlider(160, 640)
-	widthSlider.Value = float64(state.thumbWidth)
-	widthSlider.Step = 32
-	widthSlider.OnChanged = func(val float64) {
-		state.thumbWidth = int(val)
-		widthLabel.SetText(fmt.Sprintf("Thumbnail Width: %d px", int(val)))
-	}
-
 	// Contact sheet checkbox
 	contactSheetCheck := widget.NewCheck("Generate Contact Sheet (single image)", func(checked bool) {
 		state.thumbContactSheet = checked
@@ -9567,9 +9551,10 @@ func buildThumbView(state *appState) fyne.CanvasObject {
 	})
 	contactSheetCheck.Checked = state.thumbContactSheet
 
-	// Contact sheet grid options (only show if contact sheet is enabled)
-	var gridOptions fyne.CanvasObject
+	// Conditional settings based on contact sheet mode
+	var settingsOptions fyne.CanvasObject
 	if state.thumbContactSheet {
+		// Contact sheet mode: show columns and rows
 		colLabel := widget.NewLabel(fmt.Sprintf("Columns: %d", state.thumbColumns))
 		colSlider := widget.NewSlider(2, 12)
 		colSlider.Value = float64(state.thumbColumns)
@@ -9588,16 +9573,47 @@ func buildThumbView(state *appState) fyne.CanvasObject {
 			rowLabel.SetText(fmt.Sprintf("Rows: %d", int(val)))
 		}
 
-		gridOptions = container.NewVBox(
+		totalThumbs := state.thumbColumns * state.thumbRows
+		totalLabel := widget.NewLabel(fmt.Sprintf("Total thumbnails: %d", totalThumbs))
+		totalLabel.TextStyle = fyne.TextStyle{Italic: true}
+
+		settingsOptions = container.NewVBox(
 			widget.NewSeparator(),
 			widget.NewLabel("Contact Sheet Grid:"),
 			colLabel,
 			colSlider,
 			rowLabel,
 			rowSlider,
+			totalLabel,
 		)
 	} else {
-		gridOptions = container.NewVBox()
+		// Individual thumbnails mode: show count and width
+		countLabel := widget.NewLabel(fmt.Sprintf("Thumbnail Count: %d", state.thumbCount))
+		countSlider := widget.NewSlider(3, 50)
+		countSlider.Value = float64(state.thumbCount)
+		countSlider.Step = 1
+		countSlider.OnChanged = func(val float64) {
+			state.thumbCount = int(val)
+			countLabel.SetText(fmt.Sprintf("Thumbnail Count: %d", int(val)))
+		}
+
+		widthLabel := widget.NewLabel(fmt.Sprintf("Thumbnail Width: %d px", state.thumbWidth))
+		widthSlider := widget.NewSlider(160, 640)
+		widthSlider.Value = float64(state.thumbWidth)
+		widthSlider.Step = 32
+		widthSlider.OnChanged = func(val float64) {
+			state.thumbWidth = int(val)
+			widthLabel.SetText(fmt.Sprintf("Thumbnail Width: %d px", int(val)))
+		}
+
+		settingsOptions = container.NewVBox(
+			widget.NewSeparator(),
+			widget.NewLabel("Individual Thumbnails:"),
+			countLabel,
+			countSlider,
+			widthLabel,
+			widthSlider,
+		)
 	}
 
 	// Generate button
@@ -9611,15 +9627,30 @@ func buildThumbView(state *appState) fyne.CanvasObject {
 		state.showThumbView()
 
 		go func() {
-			// Create temp directory for thumbnails
-			outputDir := filepath.Join(os.TempDir(), fmt.Sprintf("videotools_thumbs_%d", time.Now().Unix()))
+			// Create output directory in same folder as video
+			videoDir := filepath.Dir(state.thumbFile.Path)
+			videoBaseName := strings.TrimSuffix(filepath.Base(state.thumbFile.Path), filepath.Ext(state.thumbFile.Path))
+			outputDir := filepath.Join(videoDir, fmt.Sprintf("%s_thumbnails", videoBaseName))
 
 			generator := thumbnail.NewGenerator(platformConfig.FFmpegPath)
+
+			// Configure based on mode
+			var count, width int
+			if state.thumbContactSheet {
+				// Contact sheet: count is determined by grid, use default width
+				count = state.thumbColumns * state.thumbRows
+				width = 320 // Fixed width for contact sheets
+			} else {
+				// Individual thumbnails: use user settings
+				count = state.thumbCount
+				width = state.thumbWidth
+			}
+
 			config := thumbnail.Config{
 				VideoPath:     state.thumbFile.Path,
 				OutputDir:     outputDir,
-				Count:         state.thumbCount,
-				Width:         state.thumbWidth,
+				Count:         count,
+				Width:         width,
 				Format:        "jpg",
 				Quality:       85,
 				ContactSheet:  state.thumbContactSheet,
@@ -9681,24 +9712,30 @@ func buildThumbView(state *appState) fyne.CanvasObject {
 	settingsPanel := container.NewVBox(
 		widget.NewLabel("Settings:"),
 		widget.NewSeparator(),
-		countLabel,
-		countSlider,
-		widthLabel,
-		widthSlider,
-		widget.NewSeparator(),
 		contactSheetCheck,
-		gridOptions,
+		settingsOptions,
 		widget.NewSeparator(),
 		generateBtn,
 	)
 
-	// Main content
+	// Main content - split layout with preview on left, settings on right
+	leftColumn := container.NewVBox(
+		videoContainer,
+	)
+
+	rightColumn := container.NewVBox(
+		settingsPanel,
+	)
+
+	mainContent := container.NewHSplit(leftColumn, rightColumn)
+	mainContent.Offset = 0.55 // Give more space to preview
+
 	content := container.NewBorder(
 		container.NewVBox(instructions, widget.NewSeparator(), fileLabel, container.NewHBox(loadBtn, clearBtn)),
 		nil,
 		nil,
 		nil,
-		settingsPanel,
+		mainContent,
 	)
 
 	return container.NewBorder(topBar, nil, nil, nil, content)