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/components/app-footer.css

@@ -0,0 +1,106 @@
+@layer components {
+  /* Site footer — utility nav with copyright, legal links, language switcher.
+     Shared between default shell, login, and public page layouts. Mobile-first. */
+  footer.site-footer {
+    border-top: 1px solid var(--app-muted-border-color);
+    padding-block: var(--app-spacing);
+    padding-inline: var(--app-spacing);
+    color: var(--app-muted-color);
+  }
+
+  footer.site-footer > nav {
+    display: flex;
+    flex-wrap: wrap;
+    align-items: center;
+    justify-content: space-between;
+    gap: calc(var(--app-spacing) * 0.5) var(--app-spacing);
+    overflow: visible;
+  }
+
+  footer.site-footer ul {
+    display: flex;
+    flex-wrap: wrap;
+    align-items: center;
+    gap: calc(var(--app-spacing) * 0.25) calc(var(--app-spacing) * 0.75);
+    margin: 0;
+    padding: 0;
+    list-style: none;
+  }
+
+  footer.site-footer li {
+    margin: 0;
+    padding: 0;
+    display: inline-flex;
+    align-items: center;
+  }
+
+  /* Reset Pico's zero-width-space before-marker on nav li. */
+  footer.site-footer li::before {
+    content: none;
+  }
+
+  footer.site-footer a {
+    color: var(--app-muted-color);
+    text-decoration: none;
+    padding-inline: calc(var(--app-spacing) * 0.25);
+    padding-block: calc(var(--app-spacing) * 0.25);
+    margin: 0;
+    border-radius: var(--app-border-radius);
+    transition:
+      color var(--app-transition),
+      background-color var(--app-transition);
+  }
+
+  footer.site-footer a:hover {
+    color: var(--app-color);
+    background: color-mix(in srgb, var(--app-contrast) 6%, transparent);
+  }
+
+  footer.site-footer a:focus-visible {
+    outline: 2px solid var(--app-primary);
+    outline-offset: -2px;
+    color: var(--app-color);
+  }
+
+  footer.site-footer small {
+    font-size: var(--text-xs);
+    line-height: var(--leading-tight);
+  }
+
+  /* Language switcher (inline <select>) — compact, sits next to the links. */
+  footer.site-footer select {
+    margin: 0;
+    padding: calc(var(--app-spacing) * 0.2) calc(var(--app-spacing) * 0.5);
+    font-size: var(--text-xs);
+    height: auto;
+    min-height: 0;
+    width: auto;
+    background-position: right 0.5rem center;
+  }
+
+  /* xs — stack link group and language group vertically, centered.
+     Better readability than cramming both on one wrapped row. */
+  @media (max-width: 576px) {
+    footer.site-footer {
+      text-align: center;
+    }
+
+    footer.site-footer > nav {
+      flex-direction: column;
+      justify-content: center;
+      gap: calc(var(--app-spacing) * 0.75);
+    }
+
+    footer.site-footer ul {
+      justify-content: center;
+    }
+  }
+
+  /* Desktop — more breathing room on the sides. */
+  @media (min-width: 768px) {
+    footer.site-footer {
+      padding-block: calc(var(--app-spacing) * 1.25);
+      padding-inline: calc(var(--app-spacing) * 2);
+    }
+  }
+}