refactor: remove inline gradient editor from CSS modal, use entity picker

Replace the gradient stop editor (canvas, markers, stop list) in the
CSS editor gradient section with a simple gradient entity selector.
Gradients are now created/edited exclusively in the Gradients tab.

Fix effect and audio palette pickers to populate from gradient entities
dynamically instead of hardcoded HTML options.
Unify all gradient/palette pickers via _buildGradientEntityItems().

Also: rename "None (use own speed)" → "None (no sync)" for sync clocks.
Add i18n keys for gradient selector and missing error messages.

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

@@ -437,7 +437,7 @@
     "common.none": "None",
     "common.none_no_cspt": "None (no processing template)",
     "common.none_no_input": "None (no input source)",
-    "common.none_own_speed": "None (use own speed)",
+    "common.none_own_speed": "None (no sync)",
     "common.undo": "Undo",
     "validation.required": "This field is required",
     "bulk.processing": "Processing…",
@@ -980,6 +980,9 @@
     "color_strip.gradient.position": "Position (0.0–1.0)",
     "color_strip.gradient.bidir.hint": "Add a second color on the right side of this stop to create a hard edge in the gradient.",
     "color_strip.gradient.min_stops": "Gradient must have at least 2 stops",
+    "color_strip.gradient.select": "Gradient:",
+    "color_strip.gradient.select.hint": "Select a gradient from the library. Create and edit gradients in the Gradients tab.",
+    "color_strip.gradient.error.no_gradient": "Please select a gradient",
     "color_strip.gradient.preset": "Preset:",
     "color_strip.gradient.preset.hint": "Load a predefined gradient palette. Selecting a preset replaces the current stops.",
     "color_strip.gradient.preset.custom": "— Custom —",