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/templates/style.css

@@ -440,6 +440,7 @@ h3 {
     text-decoration: none;
     color: #fff;
     font-family: inherit;
+    background: rgba(255,255,255,0.1);
 }
 .btn:hover { transform: translateY(-1px); }
 .btn:active { transform: scale(.97); }