feat(handlers): add file icons with standard and retro variants

Introduce file icon support to the file browser. Icons are loaded on
startup and mapped based on file name and content type.

- Load file icon mappings in the App handler initialization.
- Add `HasThumbnail`, `IconURL`, and `IconRetroURL` to the file view.
- Update CSS to support displaying file icons alongside thumbnails.
- Add retro theme support to swap standard icons with pixelated retro
  variants when the retro theme is active.

backend/libs/handlers/download.go

@@ -45,6 +45,9 @@ type fileView struct {
 	URL          string
 	DownloadURL  string
 	ThumbnailURL string
+	HasThumbnail bool
+	IconURL      string
+	IconRetroURL string
 	ReactURL     string
 	Reactions    []reactionView
 	Reacted      bool
@@ -350,6 +353,7 @@ func (a *App) fileView(box services.Box, file services.File) fileView {
 }
 
 func (a *App) fileViewWithReactions(box services.Box, file services.File, reactions []services.ReactionSummary, reacted bool) fileView {
+	icon := a.fileIcons.lookup(file.Name, file.ContentType)
 	return fileView{
 		ID:           file.ID,
 		Name:         file.Name,
@@ -359,6 +363,9 @@ func (a *App) fileViewWithReactions(box services.Box, file services.File, reacti
 		URL:          fmt.Sprintf("/d/%s/f/%s", box.ID, file.ID),
 		DownloadURL:  fmt.Sprintf("/d/%s/f/%s/download", box.ID, file.ID),
 		ThumbnailURL: fmt.Sprintf("/d/%s/thumb/%s", box.ID, file.ID),
+		HasThumbnail: file.Thumbnail != "",
+		IconURL:      fileIconURL("standard", icon.Standard),
+		IconRetroURL: fileIconURL("retro", icon.Retro),
 		ReactURL:     fmt.Sprintf("/d/%s/f/%s/react", box.ID, file.ID),
 		Reactions:    a.reactionViews(reactions),
 		Reacted:      reacted,