Add pluggable postprocessing filter system with collapsible UI

Replace hardcoded gamma/saturation/brightness fields with a flexible
filter pipeline architecture. Templates now contain an ordered list of
filter instances, each with its own options schema. Filters operate on
full images before border extraction.

- Add filter framework: base class, registry, image pool, filter instance
- Implement 6 built-in filters: brightness, saturation, gamma, downscaler, pixelate, auto crop
- Move smoothing from PP templates to device stream settings (temporal, not spatial)
- Add GET /api/v1/filters endpoint for available filter types
- Dynamic filter UI in template modal with add/remove/reorder/collapse
- Replace camera icon with display icon for screen capture streams
- Legacy migration: existing templates auto-convert flat fields to filter list

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

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

@@ -230,15 +230,23 @@
   "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.description": "Processing templates define image filters and color correction. 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:",
+  "filters.select_type": "Select filter type...",
+  "filters.add": "Add Filter",
+  "filters.remove": "Remove",
+  "filters.move_up": "Move Up",
+  "filters.move_down": "Move Down",
+  "filters.empty": "No filters added. Use the selector below to add filters.",
+  "filters.brightness": "Brightness",
+  "filters.saturation": "Saturation",
+  "filters.gamma": "Gamma",
+  "filters.downscaler": "Downscaler",
+  "filters.pixelate": "Pixelate",
+  "filters.auto_crop": "Auto Crop",
   "postprocessing.description_label": "Description (optional):",
   "postprocessing.description_placeholder": "Describe this template...",
   "postprocessing.created": "Template created successfully",
@@ -262,5 +270,7 @@
   "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.stream_settings.smoothing": "Smoothing:",
+  "device.stream_settings.smoothing_hint": "Temporal blending between frames (0=none, 1=full). Reduces flicker.",
   "device.tip.stream_selector": "Configure picture stream and LED projection settings for this device"
 }