fix(frontend): correct My Threads unread badge

[hmans]

Why

“My Threads” could show false or stale unread dots for unfollowed, muted, inaccessible, or concurrently updated threads, while the first realtime fix made every reply perform an exhaustive per-viewer followed-thread read.

What changed

• Drive the badge from accessible, unmuted, followed-and-unread thread viewer state without changing channel badges or notification routing.
• Keep live reply work bounded: a created reply/mention transition marks only its exact cached followed thread unread, while complete replacements remain limited to bootstrap/catch-up, follow/read, notification-preference, and access-restoration transitions.
• Purge thread state on access loss, restore durable follows after rejoin/member-add/universal access, and revision-fence delayed viewer refreshes.
• Preserve the exact thread target across concurrent cross-device notification dismissal, and add frontend, mounted component, core aggregate, realtime, reconnect, and access-transition regressions.
• Update the realtime architecture inventory, generated protocol reference, and repository realtime-race guidance.

Closes #1643.

API compatibility

• Additive public realtime change plus compatible behavioural changes; no ConnectRPC, auth, discovery, admin, or public HTTP shape changed.

Older client → newer server: Older clients ignore the two optional notification-transition fields and retain their existing notification-based badge behavior; known projection operations and protocol version remain unchanged.

Newer client → older server: The client falls back to resolving the target from the pending notification page; if another device dismisses it before mapping, the badge may defer until another thread-state replacement or reconnect. Servers without the access-restoration replacements likewise converge on another thread transition or reconnect.

Capability discovery or minimum client/server version: Unchanged; the fields are optional metadata on an existing protocol-v2 operation, so no capability or coordinated rollout is required.

Test plan

• mise codegen-proto && mise build-api-types — passed; generated Go, TypeScript, and reference docs are committed.
• mise test-cli — frontend production build and all CLI packages passed.
• mise x -- pnpm check (from apps/frontend) — design guardrails passed; svelte-check reported 0 errors and 0 warnings.
• Focused frontend Vitest command covering viewer mapping, mounted My Threads, projection, rooms, store, notifications, and room list — 7 files and 102 tests passed.
• Targeted core and realtime tests for mute/unmute, bounded reply delivery, notification dismissal races, access restoration, and resume catch-up — passed; these paths also passed in the full CLI suite.
• Svelte autofixer ran on every changed Svelte component/module: no issues in projection.svelte.ts; rooms.svelte.ts hit an autofixer internal parser error; store.svelte.ts reported only pre-existing $effect advisories. Frontend check is clean.
• git diff --check origin/main...HEAD, generated source-link existence, and edited architecture-paragraph length checks — passed.