refactor(ui): shared FilterChipGroup; unify feed filter chips (R13 S2c step 2)

New single-select FilterChipGroup primitive on the canonical rounded-full
"Filters bar" chip look (pill + shadow-xs + press feedback), plus an exported
filterChipClass so multi-select siblings share ONE geometry source of truth.

Migrations:
- feed watch-state chips (F1, Feed.tsx) -> FilterChipGroup, testIds preserved
- feed content-type toggles restyled to filterChipClass (stay multi-select) so
  the whole toolbar row is one chip look; wrapped in their own labelled group
- feed date presets + Custom (F3, Sidebar.tsx) -> FilterChipGroup with a
  behaviour-preserving dispatch (preset toggle-off + Custom editor intact)

Adds aria-label i18n (feed.showLabel/contentLabel, sidebar.datePresetLabel;
HU+EN) and FilterChipGroup.dom.test.tsx (6 tests). Self-E2E'd: watch switch,
date preset on/off toggle, Custom opens the range editor. gate green (vitest
136, tsc/eslint-0/knip/prettier). Branch not merged.

frontend/src/i18n/locales/en/feed.json

@@ -20,6 +20,8 @@
   "videoCount_other": "{{formattedCount}} videos",
   "sortLabel": "Sort",
   "viewLabel": "View",
+  "showLabel": "Watch state",
+  "contentLabel": "Content types",
   "view": {
     "cards": "Large cards",
     "cardsSmall": "Small cards",

frontend/src/i18n/locales/en/sidebar.json

@@ -66,6 +66,7 @@
     "shorts": "Shorts",
     "live": "Live / Upcoming"
   },
+  "datePresetLabel": "Upload date",
   "datePreset": {
     "24h": "24h",
     "1week": "1 week",

frontend/src/i18n/locales/hu/feed.json

@@ -20,6 +20,8 @@
   "videoCount_other": "{{formattedCount}} videó",
   "sortLabel": "Rendezés",
   "viewLabel": "Nézet",
+  "showLabel": "Megtekintési állapot",
+  "contentLabel": "Tartalomtípusok",
   "view": {
     "cards": "Nagy kártya",
     "cardsSmall": "Kis kártya",

frontend/src/i18n/locales/hu/sidebar.json

@@ -66,6 +66,7 @@
     "shorts": "Shorts",
     "live": "Élő / Hamarosan"
   },
+  "datePresetLabel": "Feltöltés dátuma",
   "datePreset": {
     "24h": "24 óra",
     "1week": "1 hét",