Add Picture Streams architecture with postprocessing templates and stream test UI

Introduce Picture Stream abstraction that separates the capture pipeline into
composable layers: raw streams (display + capture engine + FPS) and processed
streams (source stream + postprocessing template). Devices reference a picture
stream instead of managing individual capture settings.

- Add PictureStream and PostprocessingTemplate data models and stores
- Add CRUD API endpoints for picture streams and postprocessing templates
- Add stream chain resolution in ProcessorManager for start_processing
- Add picture stream test endpoint with postprocessing preview support
- Add Stream Settings modal with border_width and interpolation_mode controls
- Add stream test modal with capture preview and performance metrics
- Add full frontend: Picture Streams tab, Processing Templates tab, stream
  selector on device cards, test buttons on stream cards
- Add localization keys for all new features (en, ru)
- Migrate existing devices to picture streams on startup

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

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

@@ -50,9 +50,6 @@
   "templates.config.show": "Show configuration",
   "templates.config.none": "No additional configuration",
   "templates.config.default": "Default",
-  "templates.default": "Default",
-  "templates.custom": "Custom Templates",
-  "templates.default.locked": "Default template (cannot edit/delete)",
   "templates.created": "Template created successfully",
   "templates.updated": "Template updated successfully",
   "templates.deleted": "Template deleted successfully",
@@ -195,5 +192,69 @@
   "modal.discard_changes": "You have unsaved changes. Discard them?",
   "confirm.title": "Confirm Action",
   "confirm.yes": "Yes",
-  "confirm.no": "No"
+  "confirm.no": "No",
+  "common.delete": "Delete",
+  "common.edit": "Edit",
+  "streams.title": "\uD83D\uDCFA Picture Streams",
+  "streams.description": "Picture streams define the capture pipeline. A raw stream captures from a display using a capture template. A processed stream applies postprocessing to another stream. Assign streams to devices.",
+  "streams.add": "Add Picture Stream",
+  "streams.edit": "Edit Picture Stream",
+  "streams.name": "Stream Name:",
+  "streams.name.placeholder": "My Stream",
+  "streams.type": "Type:",
+  "streams.type.raw": "Screen Capture",
+  "streams.type.processed": "Processed",
+  "streams.display": "Display:",
+  "streams.capture_template": "Capture Template:",
+  "streams.target_fps": "Target FPS:",
+  "streams.source": "Source Stream:",
+  "streams.pp_template": "Processing Template:",
+  "streams.description_label": "Description (optional):",
+  "streams.description_placeholder": "Describe this stream...",
+  "streams.created": "Stream created successfully",
+  "streams.updated": "Stream updated successfully",
+  "streams.deleted": "Stream deleted successfully",
+  "streams.delete.confirm": "Are you sure you want to delete this stream?",
+  "streams.error.load": "Failed to load streams",
+  "streams.error.required": "Please fill in all required fields",
+  "streams.error.delete": "Failed to delete stream",
+  "streams.test.title": "Test Picture Stream",
+  "streams.test.run": "🧪 Run Test",
+  "streams.test.running": "Testing stream...",
+  "streams.test.duration": "Capture Duration (s):",
+  "streams.test.error.failed": "Stream test failed",
+  "postprocessing.title": "\uD83C\uDFA8 Processing Templates",
+  "postprocessing.description": "Processing templates define color correction and smoothing settings. Assign them to processed picture streams for consistent postprocessing across devices.",
+  "postprocessing.add": "Add Processing Template",
+  "postprocessing.edit": "Edit Processing Template",
+  "postprocessing.name": "Template Name:",
+  "postprocessing.name.placeholder": "My Processing Template",
+  "postprocessing.gamma": "Gamma:",
+  "postprocessing.saturation": "Saturation:",
+  "postprocessing.brightness": "Brightness:",
+  "postprocessing.smoothing": "Smoothing:",
+  "postprocessing.description_label": "Description (optional):",
+  "postprocessing.description_placeholder": "Describe this template...",
+  "postprocessing.created": "Template created successfully",
+  "postprocessing.updated": "Template updated successfully",
+  "postprocessing.deleted": "Template deleted successfully",
+  "postprocessing.delete.confirm": "Are you sure you want to delete this processing template?",
+  "postprocessing.error.load": "Failed to load processing templates",
+  "postprocessing.error.required": "Please fill in all required fields",
+  "postprocessing.error.delete": "Failed to delete processing template",
+  "postprocessing.config.show": "Show settings",
+  "device.button.stream_selector": "Stream Settings",
+  "device.stream_settings.title": "📺 Stream Settings",
+  "device.stream_selector.label": "Assigned Picture Stream:",
+  "device.stream_selector.hint": "Select a picture stream that defines what this device captures and processes",
+  "device.stream_selector.none": "-- No stream assigned --",
+  "device.stream_selector.saved": "Stream settings updated",
+  "device.stream_settings.border_width": "Border Width (px):",
+  "device.stream_settings.border_width_hint": "How many pixels from the screen edge to sample for LED colors (1-100)",
+  "device.stream_settings.interpolation": "Interpolation Mode:",
+  "device.stream_settings.interpolation.average": "Average",
+  "device.stream_settings.interpolation.median": "Median",
+  "device.stream_settings.interpolation.dominant": "Dominant",
+  "device.stream_settings.interpolation_hint": "How to calculate LED color from sampled pixels",
+  "device.tip.stream_selector": "Configure picture stream and LED projection settings for this device"
 }