Linked Issue Or Context

Closes #347

Summary

TaskComposer seeded its React state with hardcoded demo content (a fake task 为登录页添加深色模式切换 plus ~/projects/login-page / http://localhost:5173), so a freshly launched desktop app greeted users with pre-filled fake content and the textarea's placeholder never showed. This is the same class of leftover as the #344 footer cleanup, missed in the composer.

Changes (single file, apps/desktop/src/renderer/components/TaskComposer.tsx):

• Default task, workspacePath, browserUrl to empty strings so placeholders show on first launch.
• Add placeholders to the workspace (~/projects/your-app) and URL (http://localhost:5173（可选）) inputs, which previously had none.
• Add a canRun guard (task and workspace both non-empty) used by both submit() and the run button's disabled state, so the now-empty workspace default cannot launch an invalid run. browserUrl stays optional.

Impact Scope

Renderer-only, leaf component. No store, IPC, or contract changes.

GitNexus Impact Summary

• Risk level: LOW
• Critical skeleton changes: none (leaf renderer component)
• GitNexus impact: detect_changes → changed symbols TaskComposer + submit (same file), 1 affected process App → Submit (steps 2–3) — exactly the expected scope. impact(TaskComposer, upstream) → LOW, 1 direct caller (App), 1 process, 1 module (Components).
• Verification: see below.

Verification

• pnpm --filter @frontagent/desktop typecheck — clean.
• pnpm --filter @frontagent/desktop test — 13 files, 96 tests passing.
• biome check on the file — clean (formatted).
• pnpm quality:precommit ran via pre-commit hook — lint shows only the pre-existing useTemplate info in hallucination-guard (unrelated), typecheck/test green.

Note on tests: apps/desktop follows an established no-DOM-test architecture (logic lives in the store layer; components are not unit-tested). Per the repo-guard issue note, the verification intent (empty initial values + placeholders) is covered by the typecheck + the contained diff rather than a new component test, to avoid breaking that convention. Happy to add a lightweight render assertion if maintainers prefer.

Checklist

• I have linked an issue or explained why this PR stands alone.
• I have kept the diff focused on the stated change.
• I have run pnpm quality:precommit, or explained why it could not run.
• I have run pnpm quality:local for critical skeleton changes, or explained why it could not run. — N/A, not a critical skeleton change.
• I have updated docs or tests when behavior, public APIs, or Harness contracts changed. — no public API/contract change; test convention noted above.
• For critical skeleton changes, I have filled the GitNexus impact summary with concrete results. — filled despite being non-critical.

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