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/partials/plugin_config.html

@@ -893,6 +893,12 @@
                 <p class="mt-1 text-sm text-gray-600">{{ plugin.description or 'Plugin configuration' }}</p>
             </div>
             <div class="flex items-center space-x-4">
+                <button type="button"
+                        onclick="window.previewPluginNow('{{ plugin.id }}')"
+                        class="btn bg-blue-600 hover:bg-blue-700 text-white px-3 py-1.5 text-sm rounded-md"
+                        title="Run this plugin on the display for 60 seconds and open the live preview">
+                    <i class="fas fa-play mr-1"></i>Preview on display
+                </button>
                 <label class="flex items-center cursor-pointer">
                     <input type="checkbox"
                            id="plugin-enabled-{{ plugin.id }}"