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/cmd/controller/main.go

@@ -102,8 +102,8 @@ func main() {
 	if metricsStore != nil {
 		defer metricsStore.Close()
 		metricsHDDPath := cfg.Paths.HDDPath
-		if paths := sett.GetStoragePaths(); len(paths) > 0 {
-			metricsHDDPath = paths[0].Path
+		if p := sett.GetDefaultStoragePath(); p != "" {
+			metricsHDDPath = p
 		}
 		metricsCollector := metrics.NewMetricsCollector(metricsStore, cpuCollector, metricsHDDPath, logger)
 		metricsCollector.Start(ctx)