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/BulkEditorPanel.vue

@@ -186,7 +186,9 @@ async function onDeleteConfirm(token) {
 <style scoped>
 .fc-bulk-panel {
   position: fixed; top: 0; right: 0;
-  width: 320px; height: 100vh; z-index: 1100;
+  /* min(320px, 90vw): on phones the panel never swallows the whole screen —
+     leaves a sliver of the gallery visible behind it. */
+  width: min(320px, 90vw); height: 100vh; z-index: 1100;
   background: rgb(var(--v-theme-surface));
   border-left: 1px solid rgb(var(--v-theme-surface-light));
   box-shadow: -2px 0 16px rgba(0, 0, 0, 0.4);