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/layout.html

@@ -22,7 +22,7 @@
         <div class="sidebar-bottom">
             <a href="/settings" class="sidebar-settings-link {{if eq .Page "settings"}}active{{end}}">⚙ Beállítások</a>
             <div class="sidebar-footer">
-                <span class="version">v{{.Version}}</span>
+                <span class="version">{{.Version}}</span>
                 {{if .AuthEnabled}}<a href="/logout" class="logout-link">Kijelentkezés ↗</a>{{end}}
             </div>
         </div>