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/settings.html

@@ -58,7 +58,7 @@
         </div>
         <div class="settings-row">
             <span class="settings-label">Controller verzió</span>
-            <span class="settings-value mono">v{{.Version}}</span>
+            <span class="settings-value mono">{{.Version}}</span>
         </div>
     </div>
 </div>