feat(web): mobile navigation drawer + responsive CSS gap fixes

Phones previously got the desktop layout squeezed: ~12 system tabs plus one
tab per installed plugin wrapped into many rows of small pill buttons, and
the header's settings-search and system-stats widgets were dropped entirely
(hidden below their breakpoints, never relocated).

- Off-canvas nav drawer below md: the existing nav markup (system tab row +
  #plugin-tabs-row, including dynamically injected plugin tabs) is wrapped in
  a #site-nav container that CSS repositions into a slide-in drawer on small
  screens. Same DOM nodes, same @click handlers, nothing duplicated. Tabs
  become full-width rows with 44px+ touch targets. A hamburger button
  (md:hidden) in the header and a backdrop toggle the new mobileNavOpen
  Alpine state (added to both app() definitions, mirroring activeTab).
  Clicking any tab, a search result, or the backdrop closes the drawer.
  At md+ hard CSS guards make all drawer styles inert - desktop renders
  exactly as before.
- Header widgets relocated, not hidden: placeHeaderWidgets() in app.js moves
  the #settings-search-wrap and #system-stats nodes (same elements, listeners
  intact - both are looked up by id from SSE/search code, so they must never
  be duplicated) into the drawer below md and back into the header above it,
  via a matchMedia listener.
- Fixed 13 breakpoint utility classes that templates referenced but app.css
  never defined (sm:block, sm:grid-cols-2, sm:text-sm, md:block, md:w-auto,
  lg:block, lg:flex, lg:w-64, xl:grid-cols-2/3, 2xl:grid-cols-2/3/4). This
  was a live bug: 'hidden sm:block' on the search box and 'hidden lg:flex'
  on the stats meant BOTH were invisible at every screen width. Audit method
  (repeatable): diff classes used in templates vs defined in app.css.
- Mobile modal sizing: one global rule caps .modal-content at 95vw/90vh with
  internal scroll below 640px - covers every modal without per-template
  changes.
- Horizontal-scroll affordance: pure-CSS edge-fade shadows on
  .overflow-x-auto containers (scrolling-shadows technique), plus larger
  in-table touch targets below md.

Validation: breakpoint used-vs-defined audit now returns zero gaps; Jinja
parse of base.html passes; all changes to desktop behavior are additive
(new utilities) or scoped inside max-width media queries.

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

web_interface/static/v3/app.js

@@ -273,3 +273,38 @@ document.addEventListener('DOMContentLoaded', function() {
         }, 100);
     });
 });
+
+// ===== Mobile nav: header-widget relocation =====
+// Below the md breakpoint the settings-search box and system-stats block are
+// MOVED (same DOM nodes, listeners intact) from the header into the nav
+// drawer's #drawer-widgets slot; at md and up they move back. Single-instance
+// constraint: settings-search.js and the SSE stats updater both look these
+// elements up by id, so they must never be duplicated.
+window.placeHeaderWidgets = function() {
+    const drawer = document.getElementById('drawer-widgets');
+    const header = document.getElementById('header-widgets');
+    const search = document.getElementById('settings-search-wrap');
+    const stats = document.getElementById('system-stats');
+    if (!drawer || !header) return;
+
+    const desktop = window.matchMedia('(min-width: 768px)').matches;
+    if (desktop) {
+        // Restore original header order: search before the theme toggle,
+        // stats as the last item.
+        const themeToggle = document.getElementById('theme-toggle');
+        if (search && search.parentElement !== header) {
+            header.insertBefore(search, themeToggle || null);
+        }
+        if (stats && stats.parentElement !== header) {
+            header.appendChild(stats);
+        }
+    } else {
+        if (search && search.parentElement !== drawer) drawer.appendChild(search);
+        if (stats && stats.parentElement !== drawer) drawer.appendChild(stats);
+    }
+};
+
+document.addEventListener('DOMContentLoaded', function() {
+    window.placeHeaderWidgets();
+    window.matchMedia('(min-width: 768px)').addEventListener('change', window.placeHeaderWidgets);
+});