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/base/variables.base.css

@@ -81,6 +81,18 @@
   --z-drawer: 95;
   --z-drawer-backdrop: 90;
 
+  /*
+   * Responsive breakpoint system — canonical values, align all @media rules to these.
+   * CSS custom props cannot be used inside @media queries, so these are documentation only.
+   * Use them verbatim in px. Do not introduce one-off breakpoints (640, 720, 900, 968, 1100, ...).
+   *   --bp-xs:  400px  — tiny phones (iPhone SE, Pixel 4a portrait)
+   *   --bp-sm:  576px  — large phones / phablets
+   *   --bp-md:  768px  — tablets portrait, sidebar/drawer switch
+   *   --bp-lg: 1024px  — tablets landscape / small laptops, two-column details activates
+   *   --bp-xl: 1280px  — desktops
+   *   --bp-2xl: 1536px — large desktops
+   */
+
   /* Typography tokens defined in typography.tokens.css */
 }