fix(ui): mobile responsiveness — nav hamburger + primary-path fixes

The top nav packed brand + health + pipeline chip + ~7 inline links + an
action slot into one flex row, colliding/overflowing on phones (operator:
'almost unusable'). Below 768px the links now fold into a hamburger v-menu;
below 480px the brand text hides (glyph still brands). Plus the primary
browsing path:
- BulkEditorPanel: fixed 320px -> min(320px, 90vw) so it can't swallow the screen.
- GalleryFilterBar: <600px gives search its own full-width row (its 200px
  min-width was jamming the wrapping bar); sort grows.
- GalleryFacetPanel: <480px wraps groups + lets the side-by-side date inputs
  grow full-width.
- ArtistsView grid: minmax(min(440px,100%),1fr) so a card never overflows
  (single column on phones).
- GalleryView: hide the year/month timeline strip <600px.
ImageViewer already stacks its side panel below the image <900px (left as-is).

Secondary surfaces (Posts/Subscriptions filter bars, SubscriptionsTab table,
SeriesReader, PostCard) still need a mobile pass — follow-up.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/components/gallery/GalleryFacetPanel.vue

@@ -143,4 +143,11 @@ onBeforeUnmount(() => { if (debounce) clearTimeout(debounce) })
 .fc-facets__empty { color: rgb(var(--v-theme-on-surface-variant)); font-size: 0.8rem; }
 .fc-facets__date { max-width: 160px; }
 .fc-facets__dash { color: rgb(var(--v-theme-on-surface-variant)); }
+
+/* Phones: let each facet group wrap and the side-by-side date inputs grow to
+   full width so they don't overflow a ~360px viewport. */
+@media (max-width: 480px) {
+  .fc-facets__group { flex-wrap: wrap; }
+  .fc-facets__date { max-width: none; flex: 1 1 9rem; }
+}
 </style>