Add composable filter templates, skip keepalive for serial devices

Filter Template meta-filter: reference existing PP templates inside others
for composable, DRY filter chains. Filters are recursively expanded at
pipeline build time with cycle detection. New `select` option type with
dynamic choices populated by the API.

Keepalive optimization: serial devices (Adalight, AmbiLED) don't need
keepalive — they hold last frame indefinitely. Check `standby_required`
capability at processor start, skip keepalive sends for serial targets,
and hide keepalive metrics in the UI. Rename "Standby Interval" to
"Keep Alive Interval" throughout the frontend.

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

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

@@ -297,6 +297,7 @@
   "filters.auto_crop": "Auto Crop",
   "filters.flip": "Flip",
   "filters.color_correction": "Color Correction",
+  "filters.filter_template": "Filter Template",
   "postprocessing.description_label": "Description (optional):",
   "postprocessing.description_placeholder": "Describe this template...",
   "postprocessing.created": "Template created successfully",
@@ -372,8 +373,8 @@
   "targets.interpolation.dominant": "Dominant",
   "targets.smoothing": "Smoothing:",
   "targets.smoothing.hint": "Temporal blending between frames (0=none, 1=full). Reduces flicker.",
-  "targets.standby_interval": "Standby Interval:",
-  "targets.standby_interval.hint": "How often to resend the last frame when the screen is static, keeping the device in live mode (0.5-5.0s)",
+  "targets.keepalive_interval": "Keep Alive Interval:",
+  "targets.keepalive_interval.hint": "How often to resend the last frame when the source is static, keeping the device in live mode (0.5-5.0s)",
   "targets.created": "Target created successfully",
   "targets.updated": "Target updated successfully",
   "targets.deleted": "Target deleted successfully",