refactor(channels): ⋯ overflow ActionMenu + TAGS-as-trigger (R13 S3)

Declutter the channel-manager tags row:
- New reusable ActionMenu (⋯) on the AnchoredPopover platform — a compact
  overflow menu of one-shot actions with optional pending/disabled + spinner.
  The two big glass buttons (Read subscriptions, Backfill everything) move into
  it, freeing the row.
- The "YOUR TAGS" caption becomes a "✎ TAGS" trigger button that opens the tag
  editor directly, retiring the separate "Manage tags" button.

Adds i18n (channels.actionsLabel, channels.tags.label; HU+EN). ActionMenu is
built to be reused for other overflow menus later. Self-E2E'd: ⋯ opens the two
actions, ✎ TAGS opens the manager. gate green (tsc/eslint-0/prettier).

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

@@ -1,4 +1,5 @@
 {
+  "actionsLabel": "Catalog actions",
   "syncSubscriptions": "Read subscriptions from YouTube",
   "syncSubscriptionsHint": "Re-import your subscription list from YouTube — adds channels you've newly followed and drops ones you've unfollowed. The videos themselves keep syncing automatically in the background; this does not re-fetch them.",
   "backfillEverything": "Backfill everything",
@@ -45,6 +46,7 @@
     "fullySynced": "Fully synced"
   },
   "tags": {
+    "label": "Tags",
     "yourTags": "Your tags",
     "manage": "Manage tags",
     "yourTagsHint": "Your personal labels. Attach them to channels below, then filter the feed by tag from the sidebar. (Separate from the automatic language/topic tags.)",

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

@@ -45,6 +45,7 @@
     "fullySynced": "Teljesen szinkronizálva"
   },
   "tags": {
+    "label": "Címkék",
     "yourTags": "Címkéid",
     "manage": "Címkék kezelése",
     "yourTagsHint": "Saját személyes címkéid. Csatold őket az alábbi csatornákhoz, majd szűrd a hírfolyamot címke szerint az oldalsávból. (Az automatikus nyelvi/téma címkéktől függetlenül.)",