fix(desktop): remove first-send empty hero flash

[Sun-GLiang]

Summary

The first submitted prompt could share a render with the empty-chat Maka hero while its new Session was activating. This projects the optimistic row before activation, cleans it up if activation loses ownership, and stops ChatView from prepending empty content when a transient user row is already visible.

Fixes#4245

Before / after

Both images capture the first real Electron renderer mutation batch containing the submitted transient message. The input was the same image/png attachment plus 请分析这张图片中的内容 at 1280 × 800.

Before	After

Verification

• Isolated Electron E2E before/after capture: 1/1 passed on each version
• Focused first-send and streaming-handoff suites: 32/32 passed
• npm run build
• npm run lint
• npm run format:check
• npm run typecheck
• npx knip --workspace apps/desktop
• npx knip --workspace packages/ui
• git diff --check

AI use

Select exactly one:

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

Tool(s) and scope: Maka diagnosed the render-order gap, implemented the optimistic projection and empty-state guard, added regression tests, and produced the isolated Electron E2E evidence. The material commit includes a Generated-by: Maka 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

[jackwener]

Reviewed exact head a2126aa46eae3e35084f6cc90e9c5dff3b6925af.

I found no blocking issues. The first user row is now projected under the newly created Session before activation waits for observation, closing the render interval in which the empty-chat hero could appear beside the submitted prompt. The same optimistic identity is removed if activation loses ownership, the send is refused, or the operation throws. ChatView also treats any visible transient row as conversation content, so it no longer prepends the empty state while that row waits for durable or live-turn identity.

The exact-head full build and the focused first-send/streaming suites passed (32/32). A clean synthetic merge onto current main (8c491e64b35cd7af66437efc8020a88b171e2bb6) preserved the PR patch, passed the full build, and passed the same 32 tests. Biome, git diff --check, the ASF header audit, and the exact-head hosted test check also passed.

Not independently repeated: the packaged Electron screenshot capture described in the PR.

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Automated review notice: This comment was posted by an automated review agent operated by WAWQAQ. It is not an independent human review and does not replace one.