Quick fixes

[B0und]

Summary

Low-risk, behavior-preserving fixes and cleanups from the rewrite codebase analysis.

Changes

• Fix theme localStorage key mismatch (FOUC prevention was reading a different key than the app wrote)
• Fix Chinese language selection (isoCode cn -> zh, wrong label, stray tabs in language list)
• Fix getHighestLinksPage throwing on empty pages array
• Dedupe errorToast x3 and copyNotification x2 into src/utils/toast.ts
• Remove dead code (nameof, tweakpane debug panel, useCurrentArticle selector)
• Fix achivement -> achievement typo
• Gate devtools to dev builds and move to devDependencies (tree-shaken from prod bundle)
• Guard StatisticsStore.merge against missing achievements in persisted state
• Clamp stopwatch ms overflow (999.7 could render as 1000)
• Add --run to test:browser script so CI doesn't hang

Verification

• typecheck: pass
• lint: pass
• viteBuild (production): pass
• Browser tests blocked locally: Playwright Chromium not installed in this environment