feat(web): queue messages while a chat is running (outbox parity with mobile)

[radroid]

Closes #20.

Ports the mobile thread outbox to the web app. Submitting while a thread is busy or disconnected queues the message instead of erroring; a drain sends the queue FIFO once the thread is idle and connected.

What's included

• Composer UX — the primary action becomes Queue when a submit would enqueue (thread busy, environment disconnected, or the outbox is already non-empty); Stop is demoted to a secondary control while running. Queued messages show above the composer with inline edit/remove.
• Drain engine (useThreadOutboxDrain) — a single global in-flight lock, one send at a time, canSettle idle-gate (survives a just-adopted turn), settings-sync before start-turn, transient-vs-deterministic retry classification with backoff.
• Persistence — one localStorage key per queued message ("skip invalid, never fatal"); all mutations serialized through a FIFO promise chain (storage-first, then atom).
• Reuses the shared client-runtime thread-settled predicates and environment scoping — no forked logic.

Scope (matches mobile except)

• Image attachments are not queued (blob previews can't survive a localStorage reload).
• Offline new-thread creation is out of scope for v1 (existing-thread queuing only).

Pre-land adversarial review — fixed here

A 4-lens adversarial review + refutation pass found 6 findings (2 medium, 4 low). Fixed:

• [medium] Removed-environment resurrection — removing an environment now purges its queued messages (clearEnvironment, wired into EnvironmentOwnedDataCleanup), so they can't linger and auto-send if the same server is re-added. Covered by a new regression test (incl. the persisted-only "zombie" case).
• [medium] Image-only queue submit — no longer enqueues a bootstrap prompt that claims images are attached while carrying none (which also discarded the image); it warns and keeps the composer intact.
• [low] Warning ordering — the "images aren't queued" toast now fires only after a successful enqueue.
• [low] Drain heartbeat — while messages are queued, the drain re-evaluates the time-based idle-gate on a cadence so a head can't stall until an unrelated re-render.

Known v1 limitation (deferred)

• No cross-tab reconciliation — two open tabs run independent drains over the same localStorage queue (no mobile parity source; narrow scenario). Documented for a follow-up.

Verification

• tsgo --noEmit (web): 0 errors.
• vp test run src/outbox: 19 passed.
• Browser E2E is manual QA (self-tested by the author).

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