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/app.go

@@ -20,9 +20,14 @@ type App struct {
 	banService      *services.BanService
 	rateLimiter     *rateLimiter
 	uploadGroups    *uploadGrouper
+	fileIcons       *fileIconSet
 }
 
 func NewApp(cfg config.Config, logger *slog.Logger, renderer *web.Renderer, uploadService *services.UploadService, authService *services.AuthService, settingsService *services.SettingsService, reactionService *services.ReactionService, banService *services.BanService) *App {
+	fileIcons, err := loadFileIcons(cfg.StaticDir)
+	if err != nil {
+		logger.Warn("failed to load file icon map", "source", "handlers", "severity", "warn", "error", err.Error())
+	}
 	return &App{
 		cfg:             cfg,
 		logger:          logger,
@@ -34,6 +39,7 @@ func NewApp(cfg config.Config, logger *slog.Logger, renderer *web.Renderer, uplo
 		banService:      banService,
 		rateLimiter:     newRateLimiter(),
 		uploadGroups:    newUploadGrouper(),
+		fileIcons:       fileIcons,
 	}
 }