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/js/settings-search.js

@@ -173,7 +173,14 @@
 
     function setActiveTab(tab) {
         var data = getAppData();
-        if (data) { data.activeTab = tab; return true; }
+        if (data) {
+            data.activeTab = tab;
+            // Navigating from a search result should also dismiss the mobile
+            // nav drawer (harmless no-op on desktop, where the drawer CSS
+            // doesn't apply).
+            if ('mobileNavOpen' in data) data.mobileNavOpen = false;
+            return true;
+        }
         return false;
     }