[iuyo5678]

Document optional local pairing for fresh installs, existing profiles and independent CLI/extension upgrades. Installation no longer requires pairing; the CLI starts or reuses the daemon when the user chooses to pair. Explain why paired connections never automatically downgrade after verification failure.

Update installation instructions, both READMEs, privacy notices, architecture, changelog and host/remote guidance. Add a Chinese pairing guide. The UI links the existing official security overview; this PR contains the detailed pairing explanation in repository documentation.

Extension 0.4.2 introduces optional pairing and 0.4.3 removes obsolete migration code. Neither release imposes an enforcement deadline. Unpaired connections retain impersonation risk; this is optional identity verification, not a claim that unpaired access has been secured.

Validation across the final stack:

• Rust workspace: 1,020 passed, one existing ignored; formatting and strict Clippy.
• Extension: 3,570 passed, 363 environment-dependent skips; TypeScript, Biome and production build.
• Five real-daemon tests using the current CLI and published 0.4.1; real Chrome for Testing pairing/navigation smoke.
• The 0.4.2 stage independently passed 131 focused tests, three real-daemon tests, typecheck and Biome.

Stack 7/9; base: #429. Review order: #424 → #425 → #426 → #427 → #428 → #429 → #430 → #431 → #432.