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/SeriesView.vue

@@ -4,7 +4,7 @@
          the 64px TopNav (operator-asked 2026-06-12), so the axis switcher and
          search/sort stay reachable on a long grid. The controls can't sit
          inside v-window (it clips sticky children), so they're hoisted here. -->
-    <div class="fc-series__head">
+    <div class="fc-series__head fc-chrome-continues">
       <v-tabs v-model="tab" density="compact">
         <v-tab value="browse">Browse</v-tab>
         <v-tab value="suggestions">
@@ -294,12 +294,13 @@ onMounted(() => {
 
 <style scoped>
 /* Sticky header (tabs + active-tab controls) pinned under the 64px TopNav, so
-   content scrolls cleanly beneath it. Surface bg matches SettingsView. */
+   content scrolls cleanly beneath it. Background is the shared
+   .fc-chrome-continues fade — continues the nav's gradient rather than a solid
+   band (operator 2026-07-13). */
 .fc-series__head {
   position: sticky;
   top: 64px;
   z-index: 4;
-  background: rgb(var(--v-theme-surface));
   padding-bottom: 12px;
 }
 .fc-series-browse__controls {