UI polish: refresh button, negative thumbnail cache, and style fixes

- Add refresh button to browser toolbar to re-fetch current folder
- Cache "no thumbnail" results to avoid repeated slow SMB lookups
- Fix list view fallback icon sizing for files without album art
- Fix view toggle button hover (no background/scale on hover)
- Skip re-render when clicking already-active view mode button

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

media_server/static/index.html

@@ -164,6 +164,9 @@
                             <svg viewBox="0 0 24 24" width="18" height="18"><path d="M3 4h18v2H3V4zm0 7h18v2H3v-2zm0 7h18v2H3v-2z"/></svg>
                         </button>
                     </div>
+                    <button class="view-toggle-btn browser-refresh-btn" id="refreshBtn" onclick="refreshBrowser()" data-i18n-title="browser.refresh" title="Refresh">
+                        <svg viewBox="0 0 24 24" width="18" height="18"><path fill="currentColor" d="M17.65 6.35A7.958 7.958 0 0012 4c-4.42 0-7.99 3.58-7.99 8s3.57 8 7.99 8c3.73 0 6.84-2.55 7.73-6h-2.08A5.99 5.99 0 0112 18c-3.31 0-6-2.69-6-6s2.69-6 6-6c1.66 0 3.14.69 4.22 1.78L13 11h7V4l-2.35 2.35z"/></svg>
+                    </button>
                     <button class="browser-play-all-btn" id="playAllBtn" onclick="playAllFolder()" data-i18n-title="browser.play_all" title="Play All" style="display: none;">
                         <svg viewBox="0 0 24 24" width="16" height="16"><path fill="currentColor" d="M8 5v14l11-7z"/></svg>
                         <span data-i18n="browser.play_all">Play All</span>