feat(web): floating live preview + per-plugin "Preview on display", drawer a11y

Preview-while-configuring:
- Floating mini preview (fixed, bottom-right) available on every tab except
  Overview, fed from the same SSE display stream by updateDisplayPreview -
  no new connections. Collapses to a round toggle button; open/closed state
  persists in localStorage; hides on Overview where the full preview lives.
- "Preview on display" button on every plugin config page header: runs that
  plugin on the real display for 60 seconds via the existing
  /display/on-demand/start API and opens the floating preview, closing the
  configure -> see-the-result loop.

Drawer/nav accessibility:
- aria-current="page" tracks the active tab (system + dynamic plugin tabs,
  matched via their Alpine @click expression), updated from the activeTab
  watcher so search deep-links and checklist navigation are covered too.
- Escape closes the mobile drawer and returns focus to the hamburger;
  opening the drawer moves focus to its first tab.

Validation: all 40 web tests pass; Jinja parse + div balance on both
touched templates.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KEZK1P1Q1fu5pcuVrkrCFZ

web_interface/static/v3/app.css

@@ -1356,3 +1356,38 @@ button.bg-white {
 [data-theme="dark"] .power-warning-banner-dismiss {
     color: #fca5a5;
 }
+
+/* ===== Floating live preview (all tabs except Overview) ===== */
+.floating-preview {
+    position: fixed;
+    right: 1rem;
+    bottom: 1rem;
+    z-index: 70;
+    background-color: #111827;
+    border: 1px solid #374151;
+    border-radius: 0.5rem;
+    box-shadow: 0 8px 24px rgba(0, 0, 0, 0.4);
+    overflow: hidden;
+}
+.floating-preview img {
+    background-color: #000;
+}
+.floating-preview-toggle {
+    position: fixed;
+    right: 1rem;
+    bottom: 1rem;
+    z-index: 70;
+    width: 44px;
+    height: 44px;
+    border-radius: 9999px;
+    background-color: var(--color-primary);
+    color: #ffffff;
+    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.35);
+    align-items: center;
+    justify-content: center;
+}
+@media (max-width: 640px) {
+    .floating-preview img {
+        width: 192px !important;
+    }
+}