Add BetterCam engine, UI polish, and bug fixes

- Add BetterCam capture engine (DXGI Desktop Duplication, priority 4)
- Fix missing picture_stream_id in get_device endpoint
- Fix template delete validation to check streams instead of devices
- Add description field to capture engine template UI
- Default template name changed to "Default" with descriptive text
- Display picker highlights selected display instead of primary
- Fix modals closing when dragging text selection outside dialog
- Rename "Engine Configuration" to "Configuration", hide when empty
- Rename "Run Test" to "Run" across all test buttons
- Always reserve space for vertical scrollbar
- Redesign Stream Settings info panel with pill-style props
- Fix processed stream showing internal ID instead of stream name
- Update en/ru locale files

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/pyproject.toml

@@ -51,6 +51,7 @@ dev = [
 # High-performance screen capture engines (Windows only)
 perf = [
     "dxcam>=0.0.5; sys_platform == 'win32'",
+    "bettercam>=1.0.0; sys_platform == 'win32'",
     "windows-capture>=1.5.0; sys_platform == 'win32'",
 ]