web: surface OIDC session expiry instead of stalling; recover kickoff plan request - #4

… plan request

When the edge access session expires mid-build, both transports followed the
IAM redirect (or saw a 401), then returned a never-settling promise after
assign('/'). BuildPage rendered only initial-load errors, so the cached
running turn kept showing 「考えています」 indefinitely; on Craftsman this
hid a completed kickoff for 41 minutes (draft-41cb28ef, 01:56Z→02:37Z).

- One session-expiry detector for Platform proxy and Console API: 401,
  manual/opaque/followed redirects, HTML login bodies → settled
  SESSION_EXPIRED error, no retries, latch blocks further requests.
- Root-level boundary unmounts polling and shows 「セッションが切れました。
  再サインインしてください」 with 「再サインイン」 to the same draft URL.
- Kickoff recovery: a succeeded kickoff turn with no plan requests the
  generation from server history (idempotent key), and an idle unplanned
  draft offers 「この内容で提案して」 as the primary action instead of
  showing nothing.
- Active draft polling continues in background tabs.