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-shell.css

@@ -315,26 +315,12 @@
     display: block;
   }
 
-  body > footer,
   body > main {
     padding-block: var(--app-block-spacing-vertical);
   }
 
-  body > footer {
-    padding-inline: calc(var(--app-spacing) * 2);
-  }
+  /* Footer layout, spacing and colors live in components/app-footer.css. */
 
-  footer.site-footer {
-    padding-inline: calc(var(--app-spacing) * 2);
-  }
-
-  footer.site-footer a {
-    color: inherit;
-  }
-
-  body > footer a {
-    color: inherit;
-  }
   header[data-align="center"],
   hgroup[data-align="center"] > h1,
   hgroup[data-align="center"] > h2,
@@ -367,6 +353,13 @@
 
   .app-main-content {
     padding-inline: calc(var(--app-spacing) * 1);
+    padding-block-start: var(--app-spacing);
+  }
+
+  /* Details pages provide their own sticky titlebar with padding — let it manage
+     the gap to the topbar itself, don't double it. */
+  .app-main-content:has(.app-details-container) {
+    padding-block-start: 0;
   }
 
   @media (min-width: 576px) {