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/styles/app.css

@@ -61,3 +61,35 @@
   background: rgb(var(--v-theme-accent) / 0.12);
   border-radius: 8px 8px 0 0;
 }
+
+/* --- Continuous chrome fade (operator-asked 2026-07-13: "group the sub-nav as
+   part of the nav and use a single gradient in them"). ------------------------
+   The TopNav and any sticky sub-header pinned directly beneath it (Gallery's
+   filter bar, the Browse/Series/Settings/Subscriptions tabs bars) used to each
+   paint their OWN dark-to-transparent gradient (or a solid band), so the fade
+   read as happening TWICE — dark, fade out, then dark again. Instead the two
+   share ONE obsidian fade: the nav paints the TOP half (opaque → the seam
+   alpha) and the sub-header paints the CONTINUATION (seam alpha → transparent)
+   over its own height. Both reference --fc-chrome-seam, so the alphas meet
+   exactly at the 64px boundary — no re-darkening, no doubling, one gradient.
+
+   --fc-chrome-seam is the single knob: raise it for a heavier sub-header (more
+   legible tabs/controls over scrolling content), lower it for a lighter fade. */
+:root {
+  --fc-chrome-rgb: 20, 23, 26;   /* obsidian #14171A — matches the TopNav */
+  --fc-chrome-seam: 0.46;        /* alpha where the nav hands off to the sub-header */
+}
+/* Applied to a sticky sub-header so it continues the nav's fade instead of
+   restarting it. Percentage stops so the fade always spans the element's height
+   (survives the filter bar's expanding refine panel). The blur keeps tabs and
+   controls legible as the fill thins toward transparent — the solid-surface
+   bars it replaces had none, so it must live here. */
+.fc-chrome-continues {
+  background: linear-gradient(
+    to bottom,
+    rgba(var(--fc-chrome-rgb), var(--fc-chrome-seam)) 0%,
+    rgba(var(--fc-chrome-rgb), 0) 100%
+  );
+  backdrop-filter: blur(2px);
+  -webkit-backdrop-filter: blur(2px);
+}