fix(client): normalize compact composer edge spacing

[NWYLZW]

What changed

• Remove the narrow-layout 6px composer surface override so starter layouts keep the shared 10px spacing.
• Remove the new-session guide wrapper's extra 24px horizontal padding so the shared composer frame is the single horizontal spacing owner.
• Add a style contract covering both rules.

Why

Compact layouts used a local 6px override, while medium desktop windows with an expanded sidebar accumulated 24px wrapper padding plus 10px frame padding. This produced inconsistent edge spacing across responsive states.

Impact

New-session, automation, and plugin starter layouts now consistently use shared 10px surface spacing. Wide layouts retain the centered 800px maximum width.

Screenshot

Validation

• pnpm exec vitest run --workspace vitest.workspace.ts apps/client/__tests__/composer-landing.spec.tsx
• pnpm dprint check apps/client/src/routes/ChatRoute.scss apps/client/src/components/composer-landing/ComposerStarterLayout.scss apps/client/__tests__/composer-landing.spec.tsx
• Real browser checks at 973x863 with expanded and collapsed sidebar, plus 511x863 compact.
• Independent revision-bound visual review passed with no console warnings, errors, or horizontal overflow.

Experience Review

• 已判断是否需要沉淀经验
• 如需要，已运行 $post-task-experience-review
• reviewer PASS 后才进入 merge

Experience result: MERGED_WITH_EXISTING_RULE — OW-DM-001 shared 10px spacing and single-boundary ownership.

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Project	Deployment	Actions	Updated (UTC)
oneworks-dev	Ready	Preview, Comment	Jul 28, 2026 4:59pm

[github-actions]

Experience Review

Before merge, make sure the PR body contains this completed checklist:

## Experience Review
- [x] 已判断是否需要沉淀经验
- [x] 如需要，已运行 `$post-task-experience-review`
- [x] reviewer PASS 后才进入 merge

Checklist to confirm:

• Decide whether this PR produced reusable project experience.
• If experience should be captured, run $post-task-experience-review.
• Merge only after the independent reviewer reports PASS.