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/TopNav.vue

@@ -11,6 +11,8 @@
       <PipelineStatusChip />
     </div>
 
+    <!-- Desktop: inline links, centered. Hidden on mobile (see media query),
+         where they fold into the hamburger menu on the right. -->
     <nav class="fc-links">
       <RouterLink
         v-for="r in navRoutes"
@@ -20,9 +22,31 @@
       >{{ r.meta.title }}</RouterLink>
     </nav>
 
-    <!-- Per-view contextual actions teleport here (Showcase: Shuffle,
-         Gallery: Select). TopNav owns the slot, not its contents. -->
-    <div id="fc-nav-actions" class="fc-nav-actions" />
+    <div class="fc-nav-right">
+      <!-- Per-view contextual actions teleport here (Showcase: Shuffle,
+           Gallery: Select). TopNav owns the slot, not its contents. -->
+      <div id="fc-nav-actions" class="fc-nav-actions" />
+
+      <!-- Mobile nav: the link row collides with brand + actions below ~768px
+           (7+ links in one flex row), so collapse it into a menu. -->
+      <v-menu location="bottom end">
+        <template #activator="{ props: menuProps }">
+          <v-btn
+            v-bind="menuProps"
+            icon="mdi-menu" variant="text" size="small"
+            class="fc-nav-burger" aria-label="Menu"
+          />
+        </template>
+        <v-list density="compact" min-width="180">
+          <v-list-item
+            v-for="r in navRoutes"
+            :key="r.name"
+            :to="{ name: r.name }"
+            :title="r.meta.title"
+          />
+        </v-list>
+      </v-menu>
+    </div>
   </header>
 </template>
 
@@ -137,7 +161,7 @@ const health = computed(() => {
   align-items: center;
   flex-shrink: 0;
 }
-.fc-nav-actions {
+.fc-nav-right {
   flex: 1 1 0;
   min-width: 0;
   display: flex;
@@ -145,4 +169,22 @@ const health = computed(() => {
   justify-content: flex-end;
   gap: 0.5rem;
 }
+.fc-nav-actions {
+  display: flex;
+  align-items: center;
+  gap: 0.5rem;
+}
+/* The hamburger only exists on mobile; the inline links carry desktop. */
+.fc-nav-burger { display: none; }
+
+@media (max-width: 768px) {
+  .fc-topnav { gap: 0.5rem; padding: 0.6rem 0.75rem; }
+  /* Fold the link row into the hamburger menu. */
+  .fc-links { display: none; }
+  .fc-nav-burger { display: inline-flex; }
+}
+@media (max-width: 480px) {
+  /* Reclaim width on the smallest phones — the glyph alone still brands. */
+  .fc-brand__text { display: none; }
+}
 </style>