feat(responsive): mobile polish — topbar, drawer, footer, tabs, notification dropdown

Unifies the breakpoint system and fixes cross-layer CSS bugs that kept
the old mobile hide rules from ever taking effect.

Breakpoints:
- Document canonical xs/sm/md/lg/xl/2xl scale in variables.base.css
- Replace 968px one-offs with 1024px (app-details, app-page-editor)
- Make details tab-panel max-width responsive (min(70ch, 100%))

Topbar (< md):
- Hide breadcrumb in components layer so cross-layer overrides actually win
- Dock search icon alongside right-column icons via flex layout
- Add xs (400px) tightening for hit-area preservation
- Hide brand divider alongside hidden brand name
- Switch hover states to theme-aware --app-dropdown-hover-background-color

Mobile drawer:
- Icon-bar now vertical like desktop (was horizontal top strip, effectively invisible)
- Sidebar shifted 52px right so icon-bar and sidebar slide in as one 280px unit
- Add breathing room above first nav item (20px top padding)

List tabs:
- Rewrite .app-list-tabs to horizontal scroll with hidden scrollbar, snap,
  and mask-image gradient fades; add initListTabs to scroll active into view
- Register as runtime component in app-init.js

List toolbar:
- Stack wide controls (multi-select, search/text inputs) full-width below sm,
  keep compact controls at natural width

Footer:
- New dedicated app-footer.css with mobile-first stacking, border-top separator,
  theme-aware hover; remove redundant rules from app-shell.css

Notification dropdown:
- Width with min(360px, 100vw - 2rem) and min-width: 0 override global
  details > ul rule; switch to position: fixed below md so it hugs the
  viewport right edge instead of the non-rightmost bell button

Content spacing:
- Add padding-block-start to .app-main-content on mobile (was 0)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/css/layout/app-sidebar.css

@@ -153,15 +153,17 @@
     }
   }
 
+  /* Mobile drawer — sidebar sits to the right of the 52px icon-bar.
+     Both slide in together (see app-aside-icon-bar.css mobile section). */
   @media (max-width: 767px) {
     .app-sidebar {
       position: fixed;
       top: var(--app-topbar-height);
-      left: 0;
+      left: 52px;
       bottom: 0;
-      width: min(280px, 85vw);
+      width: min(228px, calc(85vw - 52px));
       z-index: var(--z-drawer);
-      transform: translateX(-100%);
+      transform: translateX(calc(-100% - 52px));
       transition: transform 300ms ease;
       background: var(--app-sidebar-background-color);
       overflow-y: auto;
@@ -172,11 +174,7 @@
     }
 
     .app-sidebar-panels {
-      padding-block-start: calc(
-        var(--app-icon-bar-item-padding, 0.6875rem) * 2
-        + var(--app-icon-bar-item-size, 22px)
-        + var(--app-icon-bar-border-width, 3px)
-      );
+      padding-block-start: calc(var(--app-spacing) * 1.25);
       padding-block-end: calc(var(--app-spacing) + env(safe-area-inset-bottom));
     }
   }