fix(ui): preserve composer paste undo history

[Sun-GLiang]

Summary

• route ordinary inline plain-text paste through the existing ChatComposerInput.onPaste seam
• use an escaped insertHTML browser transaction so paste is undone separately from immediately preceding keyboard input
• preserve the existing file, reference-sized token, token-adjacent, attachment, and IME paste paths
• add an Electron regression test covering repeated undo and plain-text fidelity for Chinese, URLs, multiline text, and HTML-special characters

Fixes #3786

Before / after evidence

Both corrected recordings run the same real Electron flow with the same minimal input: type x → paste ordinary multiline text → press undo three times. Each step displays the current composer text.

Before — upstream/main@7235069ad

The first undo removes x while leaving the pasted text. The second and third undo still leave the pasted text, reproducing #3786.

page@c2ccde11c524db300d1090fd7f645c92.webm

After — f547283a8

The first undo removes only the pasted text and leaves x; the second removes x; the third remains empty without reinsertion or duplication.

page@7da85ab9fe494f70134e34b1ff7e7118.webm

Verification

Passed locally:

• npm --workspace @maka/desktop run e2e -- composer-undo.spec.ts
• npm run lint
• npm run format:check
• npm run typecheck
• npm run check:asf-headers
• npx knip --workspace packages/ui
• npx knip --workspace apps/desktop
• npm run build

The full local npm test command reached two unrelated failures in unchanged workspaces:

• @maka/runtime: an environment-sensitive assertion expects /usr/local at executable-root index 1 even though the current Node executable already makes it the deduplicated index 0 root.
• @maka/runtime-host: cleanup of the shared ~/Library/Caches/Maka/runtime-hosts directory raced with a locally running Host and failed with ENOTEMPTY.

Current CI status

The PR CI build is currently blocked by the same regression already failing on its base commit main@689a4515f:

• base main CI failure
• PR CI failure

Both fail while building unchanged packages/runtime-host/src/__tests__/execution-host-queue.test.ts, where RuntimeHostConnection no longer exposes queryTurn, stopTurn, or startTurn. Neither packages/runtime-host nor packages/runtime differs from main in this PR. Re-running the PR workflow before main is repaired will reproduce the same failure.

AI use

Select exactly one:

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: Codex implemented the focused composer change, added the Electron regression test, ran verification, and performed code review. The commit includes the required Generated-by trailer.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No

[Astro-Han]

I reviewed this head and found no blocking issues.

The paste now uses execCommand('insertHTML') on a textContent-escaped <br> conversion to make plain-text paste a separate undo transaction, correctly leaving file/reference/IME paths unchanged.

Minor note: execCommand is deprecated but the only remaining transactional insertion primitive in this composer — consider annotating a future migration path.

No P0-P2. Checks on 7afde75d95 are blocked by base execution-host-queue failure unrelated to this change.

简体中文

该头未发现阻断，execCommand 废弃为轻度可维护性观察。