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/SubscriptionsView.vue

@@ -5,7 +5,7 @@
       align-tabs="start"
       color="accent"
       density="compact"
-      class="fc-subs-tabs"
+      class="fc-subs-tabs fc-chrome-continues"
     >
       <v-tab value="subscriptions">
         <v-icon start>mdi-account-multiple-check</v-icon>
@@ -63,7 +63,10 @@ const { tab } = useTabQuery(VALID_TABS, 'subscriptions')
 }
 .fc-subs-tabs {
   flex: 0 0 auto;
-  border-bottom: 1px solid rgb(var(--v-theme-on-surface-variant) / 0.18);
+  /* Cancel the shell's pt-2 so the tabs sit flush under the 64px nav, letting
+     the .fc-chrome-continues fade read as one gradient with it (operator
+     2026-07-13). The fade replaces the old border-bottom separator. */
+  margin-top: -8px;
 }
 .fc-subs-window {
   flex: 1 1 auto;