[2.x] feat(realtime): ambient typing dots on the index sidebar tag list

[imorland]

Extends the discussion-list ambient typing indicator (#4731) to the index sidebar tag list: a tag lights up with a presence-only dot while someone is typing in a discussion carrying it — including someone composing a new discussion in it.

Builds on #4755 (merged), which added the linkItems extension seam to TagLinkButton.

Backend (IndexTypingPresence)

• The index-typing broadcast now carries the tag IDs a typing discussion belongs to, scoped per channel so a restricted tag is never disclosed to an audience that can't see it: the public channel carries a guest-visible discussion's tags; each restricted-tag channel carries only its own tag.
• touchTags() — new-discussion compose typing. There's no discussion id yet, so the client sends its selected tag IDs on its own private-user={id} channel. The user id is taken from the (already-authorised) channel name — never the payload — and every claimed tag is re-authorised against that actor before broadcasting, so a client can't light up (and thus disclose activity in) a restricted tag it can't see.
• While typing continues, the rising edge is re-broadcast every REFRESH_MS so the dot doesn't self-clear mid-typing (the frontend TTL would otherwise lapse between coalesced bursts). Falling edge on TTL expiry, as before.

Frontend

• IndexTagTypingState — tag-keyed presence, keyed per source (discussion id, or a per-user compose key) so concurrent typers in one tag don't clear each other's dot.
• TagLinkButton renders the dot via its linkItems ItemList (the [2.x] refactor(tags): convert TagLinkButton to TSX with an extensible linkItems list #4755 seam).
• The restricted-tag channel subscriptions now come from a server-computed forum attribute (index-typing-tags) listing the restricted tags the actor can see, replacing the per-visible-tag subscribe loop — this drops the websocket-auth requests on page load from one-per-tag to one-per-visible-restricted-tag.

Reuses the existing index-typing channels, subscriptions and dot styling. No new admin settings — the existing public / restricted index-typing toggles gate it.

Testing

• yarn check-typings, yarn build (realtime): clean.
• PHP: unit 34, integration 34 — including new coverage for per-channel tag scoping, the compose-typing leak guard (a claimed-but-hidden restricted tag produces zero broadcasts), the index-typing-tags forum attribute per actor, and the rising-edge refresh/coalesce timing.
• Manually verified on a running 2.x install: dots appear/clear for replies and new-discussion composing, on both public and (visible) restricted tags, with no leak to users who can't see a tag.