feat: add 4 built-in gradients, searchable gradient picker, cleaner modal titles

- Add warm, cool, neon, pastel built-in gradients (promoted from frontend presets)
- Change gradient seeding to add missing built-ins on every startup (not just first run)
- Add searchable option to IconSelect component for filtering by name
- Enable search on gradient, effect palette, and audio palette pickers
- Simplify modal titles: "Add Gradient" / "Edit Gradient" instead of "Add Color Strip Source: Gradient"
- Update INSTALLATION.md and .env.example

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

@@ -911,8 +911,8 @@
     "aria.next": "下一个",
     "aria.hint": "显示提示",
     "color_strip.select_type": "选择色带类型",
-    "color_strip.add": "添加色带源",
-    "color_strip.edit": "编辑色带源",
+    "color_strip.add": "添加",
+    "color_strip.edit": "编辑",
     "color_strip.name": "名称：",
     "color_strip.name.placeholder": "墙壁灯带",
     "color_strip.picture_source": "图片源：",