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.lock

@@ -3973,31 +3973,6 @@ dependencies = [
  "syn 2.0.117",
 ]
 
-[[package]]
-name = "tokio-stream"
-version = "0.1.18"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "32da49809aab5c3bc678af03902d4ccddea2a87d028d86392a4b1560c6906c70"
-dependencies = [
- "futures-core",
- "pin-project-lite",
- "tokio",
- "tokio-util",
-]
-
-[[package]]
-name = "tokio-util"
-version = "0.7.18"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "9ae9cec805b01e8fc3fd2fe289f89149a9b66dd16786abd8b19cfa7b48cb0098"
-dependencies = [
- "bytes",
- "futures-core",
- "futures-sink",
- "pin-project-lite",
- "tokio",
-]
-
 [[package]]
 name = "toml"
 version = "0.8.2"
@@ -5365,7 +5340,6 @@ dependencies = [
  "serde",
  "serde_json",
  "tokio",
- "tokio-stream",
  "toml",
  "tower-http",
  "tray-icon",