fix(chat): follow-ups from the unconfirmed legacy sends review (#2164) #2172

Follow-up to #2164 (LOW notes from the merge pass's review; none blocked).

• On a PC too old to confirm messages, every chat message leaves a saved "Not confirmed" row that only a manual discard clears (src/lib/pendingMessages.ts ~248-252); intended, but a "Discard all" action could follow.
• The not-confirmed branch of the held queue's Send now (src/lib/useTerminalSends.ts ~141-143) has no unit test.
• usePaneCommands reads the cache during render without subscribing (src/lib/usePaneCommands.ts ~40); harmless identity change once the entry expires.
• Pre-existing: a PC that old ignores the resend id (ws.ts ~894-896), so two tabs pressing Send now on the same held copy can both type it.
• Recovery text lost on reload with a bridge that omits herdr_lifetime (Codex MEDIUM, src/lib/pendingMessages.ts ~248): the new uncertain row lives only in page memory, the composer clears and no unsaved warning shows, so a reload loses the text (main lost it too). Archive these copies durably in untargeted recovery storage, still guarding against restoring into a reused pane id.