feat(miniapp): refine built-in PR review workflow

[limityan]

Summary

• Refine the built-in PR Review MiniApp into a focused review workflow with source management kept out of the primary surface.
• Improve startup sync visibility, PR queue focus, diff navigation, manual comment editing, and AI review progress states.
• Add guarded lifecycle actions, safer review draft replacement, published-review context reuse, and contract tests for the built-in app assets.

Validation

• node --check src\crates\core\src\miniapp\builtin\assets\pr-review\ui.js
• rustfmt --edition 2021 --check src\crates\core\src\miniapp\builtin\mod.rs
• git diff --check -- src\crates\core\src\miniapp\builtin\assets\pr-review\style.css src\crates\core\src\miniapp\builtin\assets\pr-review\ui.js src\crates\core\src\miniapp\builtin\mod.rs
• cargo test -p bitfun-core miniapp::builtin::tests -- --nocapture
• cargo check -p bitfun-core
• pnpm run lint:web
• pnpm run type-check:web
• pnpm --dir src/web-ui exec vitest run --fileParallelism=false