Stabilize auto language detection

[bartekplus]

Summary

• replace stateless auto-detect with a session-aware coordinator that uses rolling samples, scoped live runtimes, hysteresis, session locks, and soft per-site priors
• tighten popup/options auto-detect UX with live status, waiting states, and explanatory copy for stable switching, fallback, local-only learning, and session locks
• add regression coverage for runtime scoping, rolling-sample switching, stale-session prior pruning, revisit prior behavior, and page-hint caching

Validation

• bun run lint
• bun run format:check
• bun run test
• bun run test:e2e --platform=chrome
• bun run test:e2e --platform=firefox
• bun run check:e2e:coverage
• bun run test:e2e:full --platform=chrome
• bun run test:e2e:full --platform=firefox

Notes

• bunx tsc --noEmit still fails on the existing unrelated repo-wide baseline outside this branch, including files such as src/adapters/chrome/background/PredictionOrchestrator.ts, src/adapters/chrome/content-script/suggestions/*, src/ui/options/settings*.ts, and src/ui/settings-engine/*.