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/target-editor.html

@@ -1,4 +1,4 @@
-<!-- Target Editor Modal (name, device, source, settings) -->
+<!-- Target Editor Modal (name, device, color strip source, standby) -->
 <div id="target-editor-modal" class="modal" role="dialog" aria-modal="true" aria-labelledby="target-editor-title">
     <div class="modal-content">
         <div class="modal-header">
@@ -25,48 +25,11 @@
 
                 <div class="form-group">
                     <div class="label-row">
-                        <label for="target-editor-source" data-i18n="targets.source">Picture Source:</label>
+                        <label for="target-editor-css" data-i18n="targets.color_strip_source">Color Strip Source:</label>
                         <button type="button" class="hint-toggle" onclick="toggleHint(this)" title="?">?</button>
                     </div>
-                    <small class="input-hint" style="display:none" data-i18n="targets.source.hint">Select a source that defines what to capture</small>
-                    <select id="target-editor-source"></select>
-                </div>
-
-                <div class="form-group">
-                    <div class="label-row">
-                        <label for="target-editor-fps" data-i18n="targets.fps">Target FPS:</label>
-                        <button type="button" class="hint-toggle" onclick="toggleHint(this)" title="?">?</button>
-                    </div>
-                    <small class="input-hint" style="display:none" data-i18n="targets.fps.hint">Target frames per second for capture and LED updates (10-90)</small>
-                    <div class="slider-row">
-                        <input type="range" id="target-editor-fps" min="10" max="90" value="30" oninput="document.getElementById('target-editor-fps-value').textContent = this.value">
-                        <span id="target-editor-fps-value" class="slider-value">30</span>
-                    </div>
-                </div>
-
-                <div class="form-group">
-                    <div class="label-row">
-                        <label for="target-editor-interpolation" data-i18n="targets.interpolation">Interpolation Mode:</label>
-                        <button type="button" class="hint-toggle" onclick="toggleHint(this)" title="?">?</button>
-                    </div>
-                    <small class="input-hint" style="display:none" data-i18n="targets.interpolation.hint">How to calculate LED color from sampled pixels</small>
-                    <select id="target-editor-interpolation">
-                        <option value="average">Average</option>
-                        <option value="median">Median</option>
-                        <option value="dominant">Dominant</option>
-                    </select>
-                </div>
-
-                <div class="form-group">
-                    <div class="label-row">
-                        <label for="target-editor-smoothing">
-                            <span data-i18n="targets.smoothing">Smoothing:</span>
-                            <span id="target-editor-smoothing-value">0.3</span>
-                        </label>
-                        <button type="button" class="hint-toggle" onclick="toggleHint(this)" title="?">?</button>
-                    </div>
-                    <small class="input-hint" style="display:none" data-i18n="targets.smoothing.hint">Temporal blending between frames (0=none, 1=full). Reduces flicker.</small>
-                    <input type="range" id="target-editor-smoothing" min="0.0" max="1.0" step="0.05" value="0.3" oninput="document.getElementById('target-editor-smoothing-value').textContent = this.value">
+                    <small class="input-hint" style="display:none" data-i18n="targets.color_strip_source.hint">Color strip source that captures and processes screen pixels into LED colors</small>
+                    <select id="target-editor-css"></select>
                 </div>
 
                 <div class="form-group" id="target-editor-standby-group">