fix(ui): measure real nav height (--fc-nav-h), stop Explore breadcrumb tucking under nav (#1481)

The app uses a plain sticky TopNav (no v-main), and the nav's height was
hardcoded as 64px in ~6 places: the Explore + Subscriptions full-height
workspaces (height: calc(100vh - 64px)) and every sticky sub-header pinned
beneath the nav (top: 64px — Gallery filter bar, Browse/Series/Settings tabs).

Vuetify 4's MD3 sizing changed the real nav height, so 64px was wrong: the
Explore workspace was sized taller than the space below the nav, overflowed the
viewport, and its breadcrumb tucked under the (taller) nav on 1080p.

TopNav now measures its own height via ResizeObserver and publishes it as
--fc-nav-h on documentElement (default 64px in app.css). Every consumer uses
var(--fc-nav-h) instead of the magic number, so the layout self-corrects to the
nav's real height and stays correct as it reflows (per-view teleported actions,
mobile breakpoint). Also tightens the new chrome-gradient seam — sub-headers now
pin at the nav's exact bottom.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/views/SettingsView.vue

@@ -11,7 +11,7 @@
          continues the nav's gradient across the strip (operator 2026-07-13). -->
     <v-tabs
       v-model="tab" color="accent" class="mb-4 fc-chrome-continues"
-      style="position: sticky; top: 64px; z-index: 4;"
+      style="position: sticky; top: var(--fc-nav-h, 64px); z-index: 4;"
     >
       <v-tab value="overview">Overview</v-tab>
       <v-tab value="activity">Activity</v-tab>