feat: batch .md download, multi-select, thumbnails, all OPPORTUNITIES tools

Videos view: multi-select (checkboxes) + select-all/deselect-all + bulk action
bar (Download .md batch, Download thumbnails, Download audio). Per-video buttons:
.md download (done) or Process to .md (pending), clip.

Channels: per-channel pending count + 'Download pending .md' (batches pending
video_ids). Each channel keeps Videos/.md folder/Remove actions.

Backend (jobs.py + api.py):
- POST /api/scrape/batch {video_ids} + POST /api/scrape/video/{id} (on-demand
  .md for any video incl. pending) as SSE-tracked jobs
- GET /api/videos/{id}/markdown (file download)
- POST /api/tools/thumbnails + GET /api/thumbnails/{id} (local cache, offline)
- GET /api/clip/{id}?from=&to= (transcript segment)
- GET /api/stats (aggregate totals)
- POST /api/tools/audio (mp3 job)

Tools view rebuilt as full Knowledge Base grid surfacing every OPPORTUNITIES.md
feature as electable buttons (md download, re-render, export, search, analysis,
thumbnails, audio, clip, stats, cookies, channels, watch, open folders).

Floating job widget (SSE) for any running job. Thumbnails served via
/api/thumbnails/{id} everywhere (works offline once cached).

Launcher: robust Open-Browser helper (3 fallback methods) auto-opens
http://127.0.0.1:<port> on start + on idempotent re-open. data/ gitignored.

.gitignore

@@ -7,13 +7,7 @@ dist/
 .pytest_cache/
 .coverage
 htmlcov/
-data/state.db
-data/state.db-journal
-data/state.db-wal
-data/markdown/
-data/exports/
-data/analysis/
-data/audio/
+data/
 cookies/
 .run/
 .venv/