feat: add gradient entity modal and fix color picker clipping

Add full gradient editor modal with name, description, visual stop
editor, tags, and dirty checking. Gradient editor now supports ID
prefix to avoid DOM conflicts between CSS editor and standalone modal.

Fix color picker popover clipped by template-card overflow:hidden.
Fix gradient canvas not sizing correctly in standalone modal.

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

@@ -1339,6 +1339,27 @@
     "audio_template.error.delete": "Failed to delete audio template",
     "streams.group.value": "Value Sources",
     "streams.group.sync": "Sync Clocks",
+    "streams.group.gradients": "Gradients",
+    "gradient.group.title": "Gradients",
+    "gradient.add": "Add Gradient",
+    "gradient.edit": "Edit Gradient",
+    "gradient.builtin": "Built-in",
+    "gradient.stops_label": "stops",
+    "gradient.name": "Name:",
+    "gradient.name.hint": "A descriptive name for this gradient.",
+    "gradient.description": "Description:",
+    "gradient.description.hint": "Optional description for this gradient.",
+    "gradient.created": "Gradient created",
+    "gradient.updated": "Gradient updated",
+    "gradient.cloned": "Gradient cloned",
+    "gradient.deleted": "Gradient deleted",
+    "gradient.error.name_required": "Name is required",
+    "gradient.error.min_stops": "At least 2 color stops are required",
+    "gradient.error.delete_failed": "Failed to delete gradient",
+    "gradient.create_name": "New gradient name:",
+    "gradient.edit_name": "Rename gradient:",
+    "gradient.confirm_delete": "Delete gradient \"{name}\"?",
+    "section.empty.gradients": "No gradients yet",
     "tree.group.picture": "Picture Source",
     "tree.group.capture": "Screen Capture",
     "tree.group.static": "Static",