feat: make inspector keyboard accessible - #33

Summary

• let keyboard users enter selection mode, focus a page target, and inspect it with Enter or Space
• move initial drawer focus to Why or Close, retain visible focus, and return focus to Inspect on Escape
• verify all five tabs with arrow, Home, and End navigation in a keyboard-only React 19 Playwright path
• document the keyboard workflow in English and Chinese public guides

Validation

• pnpm check
• React 19 Playwright: 8/8 passed
• keyboard test asserts active-tab semantics, visible outlines, and focus return
• privacy scan for internal identifiers and npm token patterns

Closes #25