Fix GUI background image generation race conditions

- Add file.Sync() after image download to ensure data is flushed to disk
- Add double-checking in UI updates to prevent wrong card updates
- Fix background job completion to reload files when user navigates back
- Add file size validation in image display widget
- Improve error messages for image loading failures

This fixes two issues:
1. 'png: invalid format: not enough pixel data' error when navigating during generation
2. Images not updating when navigating back to a card after background generation completes

2 files changed, 83 insertions, 13 deletions

diff --git a/internal/gui/app.go b/internal/gui/app.go
index 6a58286..685c913 100644
--- a/internal/gui/app.go
+++ b/internal/gui/app.go
@@ -716,11 +716,19 @@ func (a *Application) generateMaterials(word string) {
 		a.mu.Lock()
 		if a.currentWord == word {
 			a.currentAudioFile = audioRes.file
+			audioFile := audioRes.file
+			a.mu.Unlock()
 			fyne.Do(func() {
-				a.audioPlayer.SetAudioFile(audioRes.file)
+				// Double-check inside the UI update that we're still on the same word
+				a.mu.Lock()
+				if a.currentWord == word {
+					a.audioPlayer.SetAudioFile(audioFile)
+				}
+				a.mu.Unlock()
 			})
+		} else {
+			a.mu.Unlock()
 		}
-		a.mu.Unlock()
 	}
 
 	// Collect image result
@@ -735,11 +743,19 @@ func (a *Application) generateMaterials(word string) {
 		a.mu.Lock()
 		if a.currentWord == word {
 			a.currentImage = imageRes.file
+			imageFile := imageRes.file
+			a.mu.Unlock()
 			fyne.Do(func() {
-				a.imageDisplay.SetImages([]string{imageRes.file})
+				// Double-check inside the UI update that we're still on the same word
+				a.mu.Lock()
+				if a.currentWord == word {
+					a.imageDisplay.SetImages([]string{imageFile})
+				}
+				a.mu.Unlock()
 			})
+		} else {
+			a.mu.Unlock()
 		}
-		a.mu.Unlock()
 	}
 
 	// Collect phonetic result (UI already updated in the goroutine)
@@ -967,19 +983,34 @@ func (a *Application) onRegenerateAudio() {
 		defer a.wg.Done()
 		defer a.decrementProcessing() // Audio processing ends
 
+		// Store the word we're generating for
+		wordForGeneration := a.currentWord
+
 		// Get or create context for this card
-		cardCtx, _ := a.getOrCreateCardContext(a.currentWord)
+		cardCtx, _ := a.getOrCreateCardContext(wordForGeneration)
 
-		audioFile, err := a.generateAudio(cardCtx, a.currentWord)
+		audioFile, err := a.generateAudio(cardCtx, wordForGeneration)
 		if err != nil {
 			fyne.Do(func() {
 				a.showError(fmt.Errorf("Audio regeneration failed: %w", err))
 			})
 		} else {
-			a.currentAudioFile = audioFile
-			fyne.Do(func() {
-				a.audioPlayer.SetAudioFile(audioFile)
-			})
+			// Only update if we're still on the same word
+			a.mu.Lock()
+			if a.currentWord == wordForGeneration {
+				a.currentAudioFile = audioFile
+				a.mu.Unlock()
+				fyne.Do(func() {
+					// Double-check inside the UI update that we're still on the same word
+					a.mu.Lock()
+					if a.currentWord == wordForGeneration {
+						a.audioPlayer.SetAudioFile(audioFile)
+					}
+					a.mu.Unlock()
+				})
+			} else {
+				a.mu.Unlock()
+			}
 		}
 
 		fyne.Do(func() {
@@ -1897,6 +1928,14 @@ func (a *Application) processWordJob(job *WordJob) {
 
 		if isCurrentJob {
 			fyne.Do(func() {
+				// Double-check that we're still on the same job before updating UI
+				a.mu.Lock()
+				if a.currentJobID != job.ID {
+					a.mu.Unlock()
+					return
+				}
+				a.mu.Unlock()
+
 				a.audioPlayer.SetAudioFile(audioFile)
 				// Enable audio-related actions
 				a.regenerateAudioBtn.Enable()
@@ -1948,6 +1987,14 @@ func (a *Application) processWordJob(job *WordJob) {
 
 	if isCurrentJob {
 		fyne.Do(func() {
+			// Double-check that we're still on the same job before updating UI
+			a.mu.Lock()
+			if a.currentJobID != job.ID {
+				a.mu.Unlock()
+				return
+			}
+			a.mu.Unlock()
+
 			a.translationEntry.SetText(translation)
 			if imageFile != "" {
 				a.imageDisplay.SetImages([]string{imageFile})
@@ -2022,6 +2069,16 @@ func (a *Application) onJobComplete(job *WordJob) {
 			} else {
 				// This is a background job that completed
 				a.updateStatus(fmt.Sprintf("Background processing completed: %s", job.Word))
+
+				// Check if user has navigated back to this word
+				a.mu.Lock()
+				currentWord := a.currentWord
+				a.mu.Unlock()
+
+				if currentWord == job.Word {
+					// User is currently viewing this word, reload the files
+					a.loadExistingFiles(job.Word)
+				}
 			}
 		}
 	})
diff --git a/internal/gui/widgets.go b/internal/gui/widgets.go
index 6ad428c..df74027 100644
--- a/internal/gui/widgets.go
+++ b/internal/gui/widgets.go
@@ -73,7 +73,20 @@ func (d *ImageDisplay) SetImage(imagePath string) {
 	}
 	defer file.Close()
 
-	img, _, err := image.Decode(file)
+	// Get file info to ensure it's fully written
+	stat, err := file.Stat()
+	if err != nil {
+		d.imageLabel.SetText(fmt.Sprintf("Error getting file info: %v", err))
+		return
+	}
+
+	// If file size is 0, it might still be writing
+	if stat.Size() == 0 {
+		d.imageLabel.SetText("Image file is empty")
+		return
+	}
+
+	img, format, err := image.Decode(file)
 	if err != nil {
 		d.imageLabel.SetText(fmt.Sprintf("Error decoding image: %v", err))
 		return
@@ -83,8 +96,8 @@ func (d *ImageDisplay) SetImage(imagePath string) {
 	d.imageCanvas.Image = img
 	d.imageCanvas.Refresh()
 
-	// Update label
-	d.imageLabel.SetText(filepath.Base(imagePath))
+	// Update label with format info
+	d.imageLabel.SetText(fmt.Sprintf("%s (%s)", filepath.Base(imagePath), format))
 }
 
 // SetImages sets multiple images but only displays the first one