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/src/wled_controller/static/locales/en.json

@@ -49,7 +49,7 @@
   "templates.engine.select": "Select an engine...",
   "templates.engine.unavailable": "Unavailable",
   "templates.engine.unavailable.hint": "This engine is not available on your system",
-  "templates.config": "Engine Configuration",
+  "templates.config": "Configuration",
   "templates.config.show": "Show configuration",
   "templates.config.none": "No additional configuration",
   "templates.config.default": "Default",
@@ -67,7 +67,7 @@
   "templates.test.display.select": "Select display...",
   "templates.test.duration": "Capture Duration (s):",
   "templates.test.border_width": "Border Width (px):",
-  "templates.test.run": "\uD83E\uDDEA Run Test",
+  "templates.test.run": "\uD83E\uDDEA Run",
   "templates.test.running": "Running test...",
   "templates.test.results.preview": "Full Capture Preview",
   "templates.test.results.borders": "Border Extraction",
@@ -229,7 +229,7 @@
   "streams.error.required": "Please fill in all required fields",
   "streams.error.delete": "Failed to delete stream",
   "streams.test.title": "Test Stream",
-  "streams.test.run": "🧪 Run Test",
+  "streams.test.run": "🧪 Run",
   "streams.test.running": "Testing stream...",
   "streams.test.duration": "Capture Duration (s):",
   "streams.test.error.failed": "Stream test failed",