Introduce ColorStripSource as first-class entity

Extracts color processing and calibration out of WledPictureTarget into a
new PictureColorStripSource entity, enabling multiple LED targets to share
one capture/processing pipeline.

New entities & processing:
- storage/color_strip_source.py: ColorStripSource + PictureColorStripSource models
- storage/color_strip_store.py: JSON-backed CRUD store (prefix css_)
- core/processing/color_strip_stream.py: ColorStripStream ABC + PictureColorStripStream (runs border-extract → map → smooth → brightness/sat/gamma in background thread)
- core/processing/color_strip_stream_manager.py: ref-counted shared stream manager

Modified storage/processing:
- WledPictureTarget simplified to device_id + color_strip_source_id + standby_interval + state_check_interval
- Device model: calibration field removed
- WledTargetProcessor: acquires ColorStripStream from manager instead of running its own pipeline
- ProcessorManager: wires ColorStripStreamManager into TargetContext

API layer:
- New routes: GET/POST/PUT/DELETE /api/v1/color-strip-sources, PUT calibration/test
- Removed calibration endpoints from /devices
- Updated /picture-targets CRUD for new target structure

Frontend:
- New color-strips.js module with CSS editor modal and card rendering
- Calibration modal extended with CSS mode (css-id hidden field + device picker)
- targets.js: Color Strip Sources section added to LED tab; target editor/card updated
- app.js: imports and window globals for CSS + showCSSCalibration
- en.json / ru.json: color_strip.* and targets.section.color_strips keys added

Data migration runs at startup: existing WledPictureTargets are converted to
reference a new PictureColorStripSource created from their old settings.

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

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

@@ -343,10 +343,11 @@
   "streams.validate_image.valid": "Image accessible",
   "streams.validate_image.invalid": "Image not accessible",
   "targets.title": "⚡ Targets",
-  "targets.description": "Targets bridge picture sources to output devices. Each target references a device and a source, with its own processing settings.",
+  "targets.description": "Targets bridge color strip sources to output devices. Each target references a device and a color strip source.",
   "targets.subtab.wled": "LED",
   "targets.subtab.led": "LED",
   "targets.section.devices": "💡 Devices",
+  "targets.section.color_strips": "🎞️ Color Strip Sources",
   "targets.section.targets": "⚡ Targets",
   "targets.add": "Add Target",
   "targets.edit": "Edit Target",
@@ -358,6 +359,8 @@
   "targets.device": "Device:",
   "targets.device.hint": "Select the LED device to send data to",
   "targets.device.none": "-- Select a device --",
+  "targets.color_strip_source": "Color Strip Source:",
+  "targets.color_strip_source.hint": "Color strip source that captures and processes screen pixels into LED colors",
   "targets.source": "Source:",
   "targets.source.hint": "Which picture source to capture and process",
   "targets.source.none": "-- No source assigned --",
@@ -380,6 +383,7 @@
   "targets.delete.confirm": "Are you sure you want to delete this target?",
   "targets.error.load": "Failed to load targets",
   "targets.error.required": "Please fill in all required fields",
+  "targets.error.name_required": "Please enter a target name",
   "targets.error.delete": "Failed to delete target",
   "targets.button.start": "Start",
   "targets.button.stop": "Stop",
@@ -531,5 +535,36 @@
   "aria.cancel": "Cancel",
   "aria.previous": "Previous",
   "aria.next": "Next",
-  "aria.hint": "Show hint"
+  "aria.hint": "Show hint",
+
+  "color_strip.add": "🎞️ Add Color Strip Source",
+  "color_strip.edit": "🎞️ Edit Color Strip Source",
+  "color_strip.name": "Name:",
+  "color_strip.name.placeholder": "Wall Strip",
+  "color_strip.picture_source": "Picture Source:",
+  "color_strip.picture_source.hint": "Which screen capture source to use as input for LED color calculation",
+  "color_strip.fps": "Target FPS:",
+  "color_strip.fps.hint": "Target frames per second for LED color updates (10-90)",
+  "color_strip.interpolation": "Color Mode:",
+  "color_strip.interpolation.hint": "How to calculate LED color from sampled border pixels",
+  "color_strip.interpolation.average": "Average",
+  "color_strip.interpolation.median": "Median",
+  "color_strip.interpolation.dominant": "Dominant",
+  "color_strip.smoothing": "Smoothing:",
+  "color_strip.smoothing.hint": "Temporal blending between frames (0=none, 1=full). Reduces flicker.",
+  "color_strip.brightness": "Brightness:",
+  "color_strip.brightness.hint": "Output brightness multiplier (0=off, 1=unchanged, 2=double). Applied after color extraction.",
+  "color_strip.saturation": "Saturation:",
+  "color_strip.saturation.hint": "Color saturation (0=grayscale, 1=unchanged, 2=double saturation)",
+  "color_strip.gamma": "Gamma:",
+  "color_strip.gamma.hint": "Gamma correction (1=none, <1=brighter midtones, >1=darker midtones)",
+  "color_strip.test_device": "Test on Device:",
+  "color_strip.test_device.hint": "Select a device to send test pixels to when clicking edge toggles",
+  "color_strip.leds": "LED count",
+  "color_strip.created": "Color strip source created",
+  "color_strip.updated": "Color strip source updated",
+  "color_strip.deleted": "Color strip source deleted",
+  "color_strip.delete.confirm": "Are you sure you want to delete this color strip source?",
+  "color_strip.delete.referenced": "Cannot delete: this source is in use by a target",
+  "color_strip.error.name_required": "Please enter a name"
 }