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/views/BrowseView.vue

@@ -7,7 +7,7 @@
          switcher and the search stay reachable no matter how far you scroll.
          Background uses the theme surface token so content scrolls cleanly
          under it (matches SettingsView's sticky tabs). -->
-    <div class="fc-browse__head">
+    <div class="fc-browse__head fc-chrome-continues">
       <v-container fluid class="py-0">
         <!-- Tabs and search share one row: the axis switcher on the left, the
              search field + active-scope chips on the right (operator-asked
@@ -156,7 +156,8 @@ function clearFilter(key) {
   position: sticky;
   top: 64px; /* directly under AppShell's 64px sticky TopNav */
   z-index: 4;
-  background: rgb(var(--v-theme-surface));
+  /* Background is the shared .fc-chrome-continues fade — it continues the nav's
+     gradient instead of a solid surface band (operator 2026-07-13). */
 }
 .fc-browse__bar {
   display: flex;