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/views/ArtistsView.vue

@@ -76,7 +76,9 @@ onMounted(async () => {
 }
 .fc-artists__grid {
   display: grid;
-  grid-template-columns: repeat(auto-fill, minmax(440px, 1fr));
+  /* min(440px, 100%) so a card never exceeds the viewport — on phones the
+     min-track collapses to 100% (single column) instead of overflowing. */
+  grid-template-columns: repeat(auto-fill, minmax(min(440px, 100%), 1fr));
   gap: 12px;
 }
 .fc-artists__sentinel {

frontend/src/views/GalleryView.vue

@@ -71,4 +71,9 @@ function openImage(id) {
   .fc-gallery-layout { flex-direction: column-reverse; }
   .fc-gallery-layout__sidebar { width: 100%; max-height: 200px; }
 }
+/* Phones: the year/month timeline strip eats vertical space and reads poorly
+   as a horizontal band — drop it; the gallery scroll is the primary nav here. */
+@media (max-width: 600px) {
+  .fc-gallery-layout__sidebar { display: none; }
+}
 </style>