feat(deps): vuetify 3→4 (batch 4 phase B, #1449)

Vuetify 4 is a Material-Design-3 styling refresh with revert snippets, not a
component-API overhaul. FC's exposure was small:

- Bump vuetify ^4, vite-plugin-vuetify ^2.1.0, vue ^3.5, engines node>=24.
- Restore the dropped global CSS reset (minimal reset from the upgrade guide) in
  Vuetify's own low-precedence reset layer, so FC's margin-zeroing assumptions hold.
- v-row prop→utility: 'dense' → density=compact (×3), align=center → class=align-center.
- v-snackbar: multi-line removed → min-height=68.
- v-autocomplete #item slot: item→internalItem (item now aliases raw) in TagPicker
  + GalleryFilterBar (item.raw.* → internalItem.raw.*).

ACCEPTED (cosmetic, operator reviews live per plan #158): MD3 typography
(text-body-2 ×73), non-uppercase buttons (v4 dropped the uppercase default),
MD3 elevation. CI verifies BUILD only — the LOOK is the live-review pass.

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

frontend/src/components/settings/SystemStatsCards.vue

@@ -1,5 +1,5 @@
 <template>
-  <v-row dense>
+  <v-row density="compact">
     <v-col v-for="card in cards" :key="card.label" cols="12" sm="6" md="4" lg="3" xl="2">
       <v-card class="fc-stat">
         <v-card-text>