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/SettingsView.vue

@@ -7,13 +7,11 @@
     <!-- Sticky tabs: operator-flagged 2026-05-25 — long Import / Maintenance
          panels pushed the tab strip out of the viewport, forcing a scroll-
          to-top just to change tab. AppShell's TopNav is 64px sticky, so the
-         tab strip lives directly under it. Background uses the theme surface
-         token so it visually merges with the page rather than the
-         translucent v-tabs default. -->
+         tab strip lives directly under it. The .fc-chrome-continues fade
+         continues the nav's gradient across the strip (operator 2026-07-13). -->
     <v-tabs
-      v-model="tab" color="accent" class="mb-4"
-      style="position: sticky; top: 64px; z-index: 4;
-             background: rgb(var(--v-theme-surface));"
+      v-model="tab" color="accent" class="mb-4 fc-chrome-continues"
+      style="position: sticky; top: 64px; z-index: 4;"
     >
       <v-tab value="overview">Overview</v-tab>
       <v-tab value="activity">Activity</v-tab>