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/templates/modals/calibration.html

@@ -8,6 +8,16 @@
         </div>
         <div class="modal-body">
             <input type="hidden" id="calibration-device-id">
+            <input type="hidden" id="calibration-css-id">
+            <!-- Device picker shown in CSS calibration mode for edge testing -->
+            <div id="calibration-css-test-group" class="form-group" style="display:none; margin-bottom: 12px; padding: 0 4px;">
+                <div class="label-row">
+                    <label for="calibration-test-device" data-i18n="color_strip.test_device">Test on Device:</label>
+                    <button type="button" class="hint-toggle" onclick="toggleHint(this)" title="?" data-i18n-aria-label="aria.hint">?</button>
+                </div>
+                <small class="input-hint" style="display:none" data-i18n="color_strip.test_device.hint">Select a device to send test pixels to when clicking edge toggles</small>
+                <select id="calibration-test-device"></select>
+            </div>
             <!-- Interactive Preview with integrated LED inputs and test toggles -->
             <div style="margin-bottom: 12px; padding: 0 24px;">
                 <div class="calibration-preview">