fix(composer): restore the prompt when a run fails after send

[juacker]

The bug (still happening despite PR #78)

When a message fails to send because the run errors — tokens exhausted, 400, 429, etc. — the typed prompt is lost: it's gone from the composer input box and from the conversation, forcing a full retype.

PR #78 already restores the prompt, but only on synchronous send errors (no connection, off-workspace route). The failures you actually hit happen asynchronously: assistant_send_message persists the message, starts the run, and returns success immediately — the composer clears. The 429/400/token-limit error surfaces later, during run execution. At that point the backend's discard_unanswered_run_input retracts the unanswered user message (emits MessageDeleted), so the prompt disappears from both places and PR #78's path never had a chance to fire.

Ironically, the backend's own discard doc-comment claimed "the typed text stays recoverable via the input history" — a recovery path that never existed in the composer.

The fix

Capture the retracted text exactly where the deletion lands, and feed it back to the composer:

1. Store (sessionStore.ts): removeMessage now detects when the removed message is a user message and stashes its (trimmed) text in recoverablePrompts[sessionId]. New clearRecoverablePrompt action.
2. Wrapper (TerminalEmulatorWrapper.tsx): subscribes to recoverablePrompts[workspaceSessionId], passes it to the composer, and clears the slot once consumed.
3. Composer (TerminalEmulator.tsx): restores the prompt into the input box via the during-render setState pattern (converges; mirrors the existing draft-swap; lint-safe vs react-hooks/set-state-in-effect), guarded by restoreFailedPrompt so it never clobbers text the user has typed since. Clears the store slot in an effect (a zustand action, not a React setter, so exempt from the effect lint).

Scope / correctness

• Image-bearing messages are unaffected: the backend preserves and auto-retries them (it doesn't delete them), so no MessageDeleted fires and this path is never invoked — correct, since images aren't recoverable from text anyway.
• Restoring again after an identical repeat failure works (the applied-marker resets when the slot clears).
• Only triggers on user message retraction; assistant placeholders and empty/whitespace messages are ignored.

Verification

• 4 new store tests: capture trims text; ignores assistant-message removal; ignores empty/whitespace; clearRecoverablePrompt works.
• tsc / eslint --max-warnings 0 / vitest (132) / vite build — all clean.

Test locally

Trigger a run failure (e.g. exhaust tokens, or a 429) with text in the composer. After the error, the typed prompt should reappear in the input box instead of being lost.