Add static image picture stream type with auto-validating UI

Introduces a new "static_image" stream type that loads a frame from a URL
or local file path, enabling LED testing with known images or displaying
static content. Includes validate-image API endpoint, auto-validation on
blur/enter/paste with caching, capture template names on stream cards,
and conditional test stats display for single-frame results.

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

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

@@ -280,5 +280,15 @@
   "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"
+  "device.tip.stream_selector": "Configure picture stream and LED projection settings for this device",
+  "streams.group.static_image": "Static Image Streams",
+  "streams.add.static_image": "Add Static Image",
+  "streams.edit.static_image": "Edit Static Image",
+  "streams.type.static_image": "Static Image",
+  "streams.image_source": "Image Source:",
+  "streams.image_source.placeholder": "https://example.com/image.jpg or C:\\path\\to\\image.png",
+  "streams.image_source.hint": "Enter a URL (http/https) or local file path to an image",
+  "streams.validate_image.validating": "Validating...",
+  "streams.validate_image.valid": "Image accessible",
+  "streams.validate_image.invalid": "Image not accessible"
 }