feat(web-ui): let /compact carry the message to submit afterwards - #3751

Summary

A manual /compact used to reject every trailing argument, so the message a user wanted to send after compacting could only be typed once the maintenance turn had finished (issue #3739).

/compact <message> now keeps the trailing text as a follow-up and submits it through the normal send path after the compaction settles.

Fixes #3739

Type and Areas

Type: feature

Areas: web UI

Motivation / Impact

Before: /compact cleared the composer, ran the compaction, and left the user to write their next message afterwards. Asking to compact and then continue took two submissions separated by the whole compaction.

After: the trailing text is remembered, and submitted as a normal user turn once compactSession() settles. On the desktop entry point that promise resolves when the maintenance turn reaches a terminal state, so the follow-up turn reads the compressed context instead of racing it.

Behaviour details:

• The follow-up is queued, never steered (sendImmediately: false). An immediate send would steer the maintenance turn while its terminal bookkeeping is still settling.
• The composer keeps its attachments (clearContextsOnSuccess: false): this send owns the follow-up text, and anything attached while compacting belongs to the user's next message.
• If the follow-up cannot be sent, its text is handed back as a draft only when the composer is still empty, and it never overwrites newer typing.
• /compact with no trailing text, and every automatic compaction, submit nothing extra — auto compaction never goes through the composer at all.
• The old "use /compact without extra arguments" guard became unreachable and was removed; chatInput.compactUsage copy now documents the follow-up, and chatInput.compactFollowUpFailed is added in en-US / zh-CN / zh-TW.

Verification

• pnpm --dir src/web-ui exec vitest run equivalent via node scripts/run-vitest.mjs run for src/flow_chat/utils plus the ChatInput* suites: 84 files / 813 tests passed, including the new compactCommand.test.ts (parser) and ChatInputCompactFollowUp.test.ts (composer wiring: no argument-rejecting gate, compaction awaited before the follow-up send, draft returned without clobbering newer input).
• ESLint on ChatInput.tsx and compactCommand.ts: no problems.
• pnpm run i18n:audit: passed, 0 warnings.
• tsc --noEmit for src/web-ui: no errors on the touched files. The run still reports pre-existing environment noise in my checkout (missing maplibre-gl / @openuidev/react-lang in the installed tree); nothing in src/flow_chat.

Not verified against a running desktop build — a manual compaction needs a configured model and a session with history. The ordering claim is derived from the desktop entry point's await semantics (ConversationCoordinator::compact_session_manually awaits the maintenance turn's terminal result).

Reviewer Notes

Remote scenarios. Remote workspace and Peer Device proxy and await compact_session on the owning host, then reuse the ordinary routed send path for the follow-up; not exercised live. Detached Dispatch is the one path worth a live check: its driver returns on acceptance, not completion, so ordering relies on the target queuing the prompt turn behind the compact turn (the target already rejects a prompt on a compact turn, so the follow-up is necessarily a separate continue). Mobile web and IM bots never go through this composer and are unchanged.

Overlap with #3588. That PR rewrites the same submitCompactFromInput (an admitted /compact must not be re-filled into the composer) for a different symptom. Whichever lands second needs a small textual merge; I can rebase on top of it if it lands first.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable.