fix(ui): one continuous chrome gradient across nav + sticky sub-headers (#1478)

The TopNav and each sticky sub-header pinned beneath it (Gallery's filter bar,
the Browse/Series/Settings/Subscriptions tabs bars) each painted their OWN
dark-to-transparent gradient (Gallery) or a solid surface band (the rest), so
the fade read as happening twice — dark, fade out, then dark again — instead of
one gradient flowing from the nav down through the sub-nav.

Operator asked to treat the sub-nav as part of the nav with a single gradient.
New shared .fc-chrome-continues primitive (app.css): the nav fades from opaque
to a shared --fc-chrome-seam alpha (on views flagged meta.stickyChrome), and the
sub-header continues from that exact seam alpha to transparent over its own
height. Both reference the same var so the alphas meet at the 64px boundary — no
re-darkening, no doubling. Percentage stops keep it spanning the filter bar's
expanding refine panel; the primitive's blur keeps tabs/controls legible where
the old solid bars had none. --fc-chrome-seam is the single tuning knob.

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

frontend/src/router.js

@@ -20,7 +20,7 @@ const routes = [
 
   // FC-2: image backbone
   { path: '/showcase', name: 'showcase', component: ShowcaseView, meta: { title: 'Showcase', navOrder: 10 } },
-  { path: '/gallery',  name: 'gallery',  component: GalleryView,  meta: { title: 'Gallery', navOrder: 20  } },
+  { path: '/gallery',  name: 'gallery',  component: GalleryView,  meta: { title: 'Gallery', navOrder: 20, stickyChrome: true } },
   // Explore: a 3-pane tagging workspace — walk an image's visual neighbours
   // (left) while tagging the focused image (center viewer + modal-parity tag
   // rail). Optional anchor param — the bare /explore nav entry SEEDS a random
@@ -29,11 +29,11 @@ const routes = [
   // Browse hub (operator-asked 2026-06-09): Posts / Artists / Tags as tabs —
   // the three "browse the library by an axis" surfaces. One nav entry; the old
   // standalone paths redirect into the matching tab (below).
-  { path: '/browse', name: 'browse', component: BrowseView, meta: { title: 'Browse', navOrder: 30 } },
+  { path: '/browse', name: 'browse', component: BrowseView, meta: { title: 'Browse', navOrder: 30, stickyChrome: true } },
   // Artist detail — no meta.title (reached by clicking an artist, not nav).
   { path: '/artist/:slug', name: 'artist', component: ArtistView },
   // Series browse — a nav entry (meta.title).
-  { path: '/series', name: 'series', component: SeriesView, meta: { title: 'Series', navOrder: 40 } },
+  { path: '/series', name: 'series', component: SeriesView, meta: { title: 'Series', navOrder: 40, stickyChrome: true } },
   // Series management — no meta.title (reached from a series card/tag).
   { path: '/series/:tagId', name: 'series-manage', component: SeriesManageView },
   // Series reader — immersive (no top nav, no meta.title).
@@ -41,10 +41,10 @@ const routes = [
 
   // FC-3: subscription backbone — purely management (sources/downloads),
   // distinct from the Browse hub.
-  { path: '/subscriptions', name: 'subscriptions', component: SubscriptionsView, meta: { title: 'Subscriptions', navOrder: 50 } },
+  { path: '/subscriptions', name: 'subscriptions', component: SubscriptionsView, meta: { title: 'Subscriptions', navOrder: 50, stickyChrome: true } },
 
   // Settings — config, pinned to the right of the nav (TopNav special-cases it).
-  { path: '/settings', name: 'settings', component: SettingsView, meta: { title: 'Settings' } },
+  { path: '/settings', name: 'settings', component: SettingsView, meta: { title: 'Settings', stickyChrome: true } },
 
   // The old standalone paths now redirect into the Browse hub, preserving any
   // deep-link query (e.g. /posts?post_id=N → /browse?tab=posts&post_id=N). The