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>

modules/notifications/web/css/components/app-notification-dropdown.css

@@ -1,6 +1,5 @@
 @layer components {
   ul.app-notification-dropdown {
-    width: 360px;
     padding: 0;
     --app-notification-text: var(--app-dropdown-color);
     --app-notification-muted: var(--app-muted-color);
@@ -9,6 +8,32 @@
     --app-notification-focus: var(--app-primary-focus);
   }
 
+  /* Dropdown sizing — must out-specify the global
+     `details.dropdown > summary + ul { width: 100%; min-width: fit-content }`
+     rule in app-shell.css, otherwise the content's intrinsic width wins and
+     the dropdown overflows the viewport on mobile. */
+  .app-header details.dropdown > summary + ul.app-notification-dropdown {
+    width: min(360px, calc(100vw - var(--app-spacing) * 2));
+    max-width: min(360px, calc(100vw - var(--app-spacing) * 2));
+    min-width: 0;
+  }
+
+  /* Below md, the bell is NOT the rightmost topbar item (bookmark / tenant /
+     account sit to the right of it). Anchoring the dropdown with `right: 0`
+     relative to the bell's <details> therefore lands its left edge off-viewport
+     on narrow screens. Switch to viewport-anchored `position: fixed` so the
+     dropdown hugs the viewport's right edge instead of the bell. */
+  @media (max-width: 767px) {
+    .app-header details.dropdown > summary + ul.app-notification-dropdown {
+      position: fixed;
+      top: var(--app-topbar-height);
+      right: var(--app-spacing);
+      left: auto;
+      inset-inline-end: var(--app-spacing);
+      inset-inline-start: auto;
+    }
+  }
+
   .app-notification-dropdown__header {
     padding: 12px 16px;
     border-bottom: 1px solid var(--app-notification-border);