refactor: remediate project audit findings

[urzeye]

Summary

• load site adapters and locale resources on demand through a shared runtime, reducing the combined extension content-entry raw size by 49.8%
• unify dialogs, menus, tooltips, loading states, prompt overlays, and queue UI under the Shadow DOM overlay root with shared theme tokens and accessibility behavior
• align Options and in-panel settings navigation, add responsive layouts and accessible switches, and remove remaining hard-coded UI copy across all 11 locales
• tighten manager cleanup, storage synchronization, URL/polling behavior, userscript mounting, and scroll-lock ownership across extension and userscript builds
• add Vitest regression coverage, zero-warning lint enforcement, theme-token validation, bundle budgets, full-platform CI builds, and updated architecture documentation

Validation

• pnpm format:check
• pnpm lint:check
• pnpm typecheck
• pnpm test:run (17 files, 41 tests)
• pnpm check:theme-tokens
• pnpm build
• pnpm build:firefox
• pnpm build:userscript
• pnpm check:bundle-budget

Audit outcomes

• extension main + ui-entry: 3,764,783 B raw, about 975.4 KiB gzip (49.8% below the audit baseline)
• userscript: 1,830,738 B raw, about 467.8 KiB gzip
• ESLint warnings: 110 -> 0
• UI changes preserve the existing information architecture and use the shared 24-theme/Shadow DOM styling path

See docs/developer/project-audit-2026-07-10.md for the per-finding remediation table.