feat(channels): leading '＋ Tag' pill + detail-page Unhide affordance (S6)

Item 6 — the tag add-trigger now LEADS the chip row (a stable left anchor)
instead of trailing it, so on the detail page (bounded=false) adding chips
grows the row to its right and the '+'/its editor popover no longer drift.
Compact icon in the width-bounded manager cell; a labelled '＋ Tag' pill
where there's free width (the detail header).

Item 7 — surface the per-subscription 'hidden' flag on the detail GET
(channel_detail was omitting it) and add an Eye/EyeOff hide/unhide toggle
next to Block on the channel page, mirroring the manager. FE ChannelDetail
gains 'hidden'; new channel.* i18n keys (HU+EN).

Tests: +3 backend (channel_detail surfaces hidden, both states + unsubscribed
default). Gate green: backend pytest 471, FE vitest 498.

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

@@ -47,6 +47,7 @@
   },
   "tags": {
     "label": "Tags",
+    "add": "Tag",
     "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.)",
     "noneYet": "No tags yet — create one below.",
     "create": "Create tag",