refactor(ui): align selection menus with shared anatomy

[GCWing]

Summary

• align shared Menu, NavigationPanel, and Web navigation row spacing with canonical design-system anatomy
• reuse shared Menu section structure and SearchField styling in the model selector and branch quick switch
• ask for confirmation before creating a new Session from a different Harness or Agent choice, with en-US, zh-CN, and zh-TW copy

Validation

• pnpm run design-system:test
• focused Web Vitest: 6 files, 52 tests
• pnpm run i18n:audit
• pnpm run motion:audit (inventory reviewed; one existing out-of-scope warning)
• pnpm run type-check:web
• pnpm run theme:visual-contract
• typography audit and Appearance contract audit passed through pnpm run check:web
• git diff --check origin/1.0.0-explore...HEAD

pnpm run check:web does not complete because theme:color-audit:all reports stale generated Git Graph MiniApp bundles and native mobile token projections outside this PR's staged paths. Those generated files were not added to this PR.

Browser visual validation and E2E were not run. Remote workspace, Remote Control, Peer Device Mode, and Detached Dispatch were not exercised because this change is limited to shared presentation and local selection behavior; it does not change transport or runtime contracts.