feat(browser): render browser "pages" stream in timeline + transcripts
- Transcripts segment_content merges browser_*.jsonl chunks into the unified timeline by epoch-ms, reports data_state.browser (presence-based "analyzed"), and routes per-file parse failures into the existing warnings channel.
- Transcripts date-nav counts browser-only days: new browser_segments day-stat (journal-stats owns stats.json), counted in _day_range_count on both the cache and raw-scan paths; stats schema bumped v7→v8.
- Timeline _build_day buckets carry has_browser + a browser_origin for mixed-bucket reachability, browser ranked below audio/screen; _load_segment returns a per-site browser field with visible per-file degrade.
- Client (both apps): browser river marks, per-site detail pane, "pages" labels/footer, transcripts browser tab/chunk rendering. Owner-facing modality name is "pages".
- Fixtures + server-side tests for the API-observable behavior; corrupt-file content seeded at test time so committed fixtures stay contract-valid.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>