Add persistent Session language switcher

[doodlewind]

What changed

• add a persistent EN | 中文 Session-language control across the public, reader, and workspace shells
• select bilingual Session titles, Summary bodies, and feed excerpts from one language snapshot
• preserve browser-locale behavior for first visits, then sync an explicit choice across routes, reloads, and tabs
• mark localized content with the language actually rendered while keeping the English product chrome unchanged
• update the design contract and add focused responsive browser coverage

User impact

Readers can explicitly switch Session content between English and Simplified Chinese without signing in. The preference stays on the device and applies consistently to Public, Team, Mine, and Session reading surfaces. Switching does not download the raw Session record history.

Validation

• pnpm check
• pnpm typecheck
• pnpm --filter @spool/web test (330 tests)
• pnpm --filter @spool/web run build
• focused Playwright language + design-system suite (8 tests at 320, 451, 768, 1024, and 1440px)