feat(channels): sidebar settings menu, channel rate limits, limits for everyone

[psamiton]

Channel settings existed but the feature was half-built: reachable only from a gear inside the channel view, rules read by nothing at all, char_limit enforced for mind senders only, and no rate limit anywhere in the repo.

What this adds

• Sidebar dots menu on channel rows, matching mind rows: Channel settings / Make private / Leave channel / Delete channel. The settings modal already existed — it just wasn't reachable without opening the channel first.
• Channel-wide rate limit (rate_limit + rate_window): at most N messages per W seconds, pooled across all senders. Over-limit sends are refused with 429 naming the limit and when it frees up. Nothing is queued or silently dropped.
• Both limits now bind every sender, not just minds, and char_limit became a per-message rather than per-text-block rule. Enforcement lives in one place, lib/chat/channel-limits.ts.
• Channels introduce themselves to minds — description, rules and limits — once per channel per session, on the hook that already pays for participant profiles. Previously a mind met a limit only by being refused by it, and never saw rules at all.
• Composer shows a live counter against the limit and blocks an over-limit send; a refused send surfaces the server's reason and hands the text back.

Behavior changes to know about

1. Editing settings tightens from any member to creator-or-admin. No schema change was needed: createConversation already stamps the creator role: "owner". A mind therefore can't lift a limit set to restrain it — but it also loses edit rights on channels it didn't create, and ownerless channels (the commons) become admin-only.
2. The channel row's X is gone. It called deleteConversation — a hard delete that destroyed the channel and its whole history for every participant, available to any participant. Replaced by an explicit two-click Delete channel, gated to owners and admins on both client and server.
3. Humans are subject to both limits, and char_limit counts the whole message.
4. Bridged channels: inbound Discord/Slack messages are deliberately not refused (that would delete words said on another platform) but they do count toward the window, so a busy bridge can throttle Volute senders. Documented in the module.

Bugs found and fixed along the way

• The settings modal called .trim() on bind:value from an <input type="number">, which yields a number — so saving a character limit always threw and silently did nothing, ever since the modal shipped.
• The X hard-delete above.
• A follow-up review caught that the sidebar-opened modal mounted before its settings fetch resolved, and the modal seeds its form once at mount — so opening settings on a channel that had them showed a blank form whose Save erased description, rules and both limits and flipped a private channel public. Both callers now load before opening.

The second commit closes ten review findings in total (rate window counting automated "event" rows; files staged before the limit check leaving orphaned pending offers; echo-text bypassing limits; a refused send destroying the typed message; leaving silently forfeiting ownership; a half-cleared rate pair silently disabling enforcement; the channel card never re-announced after a mid-session change).

Verification

• npm test 3360/3360, npm run test:e2e 70/70, tsc + svelte-check + template typecheck clean.
• Every new check was broken deliberately and the new tests confirmed red before being accepted green.
• Driven in a browser against a throwaway daemon (not the dev ~/.volute): menu, settings round-trip on a fully-populated channel, save-unchanged preserving all five settings, the 63/50 counter in red, the real rate-limit text in chat, the refused message returning to the composer, owner-leave and delete confirms, and the authz gating A/B'd by flipping ownership (as a plain member the settings items and the header gear both disappear).

Follow-ups filed rather than folded in: #890 (GET /api/v1/channels/:name has no authorization at all) and #891 (the channels.private / conversations.private drift).

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