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/templates/v3/base.html

@@ -555,6 +555,27 @@
         </nav>
         </div> <!-- /#site-nav -->
 
+        <!-- Floating mini display preview: available on every tab except
+             Overview (which has the full preview). Fed from the same SSE
+             stream by updateDisplayPreview in app-shell.js; open/closed state
+             persists in localStorage. -->
+        <div id="floating-preview" class="floating-preview" style="display:none">
+            <div class="flex items-center justify-between px-2 py-1">
+                <span class="text-xs text-gray-300"><i class="fas fa-tv mr-1"></i>Live Preview</span>
+                <button type="button" onclick="window.toggleFloatingPreview(false)"
+                        class="text-gray-400 hover:text-white ml-2" aria-label="Close live preview">
+                    <i class="fas fa-times text-xs"></i>
+                </button>
+            </div>
+            <img id="floating-preview-img" alt="Live display preview"
+                 style="image-rendering: pixelated; display: block; width: 256px; height: auto;">
+        </div>
+        <button id="floating-preview-toggle" type="button" onclick="window.toggleFloatingPreview(true)"
+                class="floating-preview-toggle" style="display:none"
+                title="Show live display preview" aria-label="Show live display preview">
+            <i class="fas fa-tv"></i>
+        </button>
+
 
         <!-- Tab content -->
         <div id="tab-content" class="space-y-6">