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/config.py

@@ -33,6 +33,7 @@ class StorageConfig(BaseSettings):
     picture_sources_file: str = "data/picture_sources.json"
     picture_targets_file: str = "data/picture_targets.json"
     pattern_templates_file: str = "data/pattern_templates.json"
+    color_strip_sources_file: str = "data/color_strip_sources.json"
     profiles_file: str = "data/profiles.json"