Fix web UI, sort labels, add channel right-click menu

Web UI:
- Replace SSE with simple 600ms polling (GET /api/progress) — SSE added
  complexity with no real benefit for a local tool and was the source of
  connection failures
- Poll task moved into request handler (dl.poll() on each /api/progress call)
  so no background goroutine needed; simplifies state greatly
- Remove tokio-stream dependency (no longer needed)
- Fix: create channels dir before opening DB in web serve()
- Rescan also refreshes watched set from DB
- Bulk watched, sort, channel download button all working in web UI

Sort labels:
- Rename "Dur ↑" / "Dur ↓" → "Shortest" / "Longest"
- Rename "Size ↑" / "Size ↓" → "Smallest" / "Largest"

Channel right-click context menu:
- Right-click any channel in sidebar → "Check for new videos" starts a
  yt-dlp job on the stored channel_url from its info.json
- Also shows "Open folder" shortcut
- Grayed out with tooltip if no URL is stored yet (channel has no info.json)

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Cargo.toml

@@ -15,7 +15,6 @@ tray-icon = "0.13"
 notify-rust = { version = "4", default-features = false, features = ["z"] }
 axum = { version = "0.7", features = ["macros"] }
 tokio = { version = "1", features = ["full"] }
-tokio-stream = { version = "0.1", features = ["sync"] }
 tower-http = { version = "0.5", features = ["cors"] }
 
 [profile.release]