fix(web,hub): surface inactive-session error on text-only send (closes #918)

[heavygee]

Summary

Closes #918.

Sending text via the web composer to an archived/inactive session
silently dropped on the floor: the hub returned 409 but the web client
swallowed the failure in resolveSessionId's catch branch with only a
console.error, leaving the operator with no toast, no inline failed-bubble,
and no Reopen affordance.

This change closes the visibility hole and adds a one-click recovery, with
no regression to the existing 5xx / network text-restore UX.

Changes

Hub (one line + a test): requireSession({requireActive: true})'s 409
body now carries a machine-readable code: 'session_inactive' next to the
existing error string. Lets the web client discriminate this branch from
other 4xx without string-matching the i18n-able human message.

Web:

1. useSendMessage now fires onError on resolveSessionId rejection,
   not just on POST /messages failure. The mutation never started, but
   the operator still needs to see the typed text retained and a reason
   surfaced.

2. The route component (router.tsx) classifies the thrown error: a 409
   with code: 'session_inactive', or a synthetic ApiError thrown from
   resolveSessionId, attaches a Reopen action to the existing inline
   composer-error affordance. Plain 4xx / 5xx / network keep the legacy
   text-restore UX untouched.

3. The Reopen click calls api.reopenSession (matches SessionList's
   menu item), invalidates the session queries, and navigates to the
   resumed session id. Per the discovery brief's friction-pass on POST /reopen on archived cursor ACP session: spawned cursor-agent dies ~90s after successful ACP load, merges away the original (no recovery) #917
   (broken reopen path), it does not auto-replay the send -- the
   operator re-clicks Send on the restored composer text once Reopen
   lands. This keeps the failure-mode blast radius identical to the
   existing Reopen surface.

4. HappyComposer's inline error region now renders an optional action
   button (data-testid='composer-send-error-action').

5. chat.sendError.sessionInactive + chat.sendError.sessionInactive.action
   added to en.ts and zh-CN.ts.

Test plan

• Hub test: messages.test.ts -- new test verifies the 409 body
  carries code: 'session_inactive'.
• Hook tests: useSendMessage.test.tsx -- new tests cover
  `POST returns ApiError(409, 'session_inactive')` (onError carries
  the typed ApiError), `resolveSessionId rejects` (onError fires
  keyed by the original sessionId, closing the silent-drop hole), and
  `5xx still uses the legacy text-restore path` (no regression --
  code is null, classifier falls through to the verbatim error
  message).
• `bun typecheck` + `bun run test:hub` + `cd web && bun run test`
  all green locally.
• Playwright smoke against the soup driver: mocked the 409+code
  response, sent from the composer, asserted inline error + Reopen
  button visible within 1s of click. Screenshot in
  `localdocs/playwright-runs/peer-b-918-handoff.png` (operator-local,
  not committed).

Out of scope

• The wider hub-restart-cascade bundle (cursor ACP fresh-session: cursorSessionId persistence race during hub restart strands session #913, cli/runnerLifecycle.ts: default archiveReason "User terminated" applied to SIGTERM-from-hub-restart archives, polluting audit trail #914, architecture: hub restart cascade-archives every idle agent session; sessions should be resilient to hub bouncing #915, POST /api/sessions/:id/archive returns 500 when CLI is gone (rpcGateway.killSession throws unhandled) #916, POST /reopen on archived cursor ACP session: spawned cursor-agent dies ~90s after successful ACP load, merges away the original (no recovery) #917,
  sessionCache: renameSession / clearSessionArchiveMetadata / restoreSessionArchiveMetadata throw on version-mismatch without refresh; forever-409 until out-of-band cache refresh #919) is being handled by a sibling peer on a separate worktree.
• hub: mergeSessionData transfers a fixed field list then cascade-deletes the old session row, silently dropping any FK-tied data not enumerated (blocks scratchlist v2 #893 / migrate-on-delete #894 data-preservation guarantees) #920 (mergeSessionData data loss) is a different code path.
• The POST /reopen on archived cursor ACP session: spawned cursor-agent dies ~90s after successful ACP load, merges away the original (no recovery) #917 reopen-path fragility isn't fixed here -- this PR routes
  through the existing Reopen endpoint as-is. If Reopen fails the route
  shows a toast with the underlying message (mirrors SessionList's
  Reopen behavior).

AI disclosure

Implemented by an AI agent (Claude Opus 4.7) acting on operator
instructions per CONTRIBUTING.md. Operator reviewed the patch and the
Playwright smoke output before opening this PR.

Made with Cursor

[github-actions]

Findings

• None.

Summary

• Review mode: initial
• No issues found in the latest diff. Residual risk: local validation could not be run in this runner.

Testing

• Not run: bun is not installed in this runner (bun: command not found).

HAPI Bot