v0.12.1: fix vv version display, HDD default path, backup status colors, button contrast

- Bug 1: Remove hardcoded 'v' prefix from templates (layout.html, settings.html); version tag already contains 'v'
- Bug 2: primaryHDDPath() and metrics collector now use GetDefaultStoragePath() instead of paths[0].Path so the real HDD is used, not the first (SSD) path
- Bug 3: Apps without HDD data show green/yellow based on volumeLastStatus instead of always gray
- Bug 5: Add default background rgba(255,255,255,0.1) to .btn to fix white-on-transparent readability

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

controller/internal/web/server.go

@@ -150,10 +150,10 @@ func (s *Server) ServeStorageAPI(w http.ResponseWriter, r *http.Request) {
 	s.storageAPIHandler(w, r)
 }
 
-// primaryHDDPath returns the first registered storage path, or the legacy config value.
+// primaryHDDPath returns the default storage path, or the legacy config value.
 func (s *Server) primaryHDDPath() string {
-	if paths := s.settings.GetStoragePaths(); len(paths) > 0 {
-		return paths[0].Path
+	if p := s.settings.GetDefaultStoragePath(); p != "" {
+		return p
 	}
 	return s.cfg.Paths.HDDPath
 }