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/templates/modals/css-editor.html

@@ -2,7 +2,7 @@
 <div id="css-editor-modal" class="modal" role="dialog" aria-modal="true" aria-labelledby="css-editor-title">
     <div class="modal-content">
         <div class="modal-header">
-            <h2 id="css-editor-title"><svg class="icon" viewBox="0 0 24 24"><rect width="18" height="18" x="3" y="3" rx="2"/><path d="M7 3v18"/><path d="M3 7.5h4"/><path d="M3 12h18"/><path d="M3 16.5h4"/><path d="M17 3v18"/><path d="M17 7.5h4"/><path d="M17 16.5h4"/></svg> <span data-i18n="color_strip.add">Add Color Strip Source</span></h2>
+            <h2 id="css-editor-title"><svg class="icon" viewBox="0 0 24 24"><rect width="18" height="18" x="3" y="3" rx="2"/><path d="M7 3v18"/><path d="M3 7.5h4"/><path d="M3 12h18"/><path d="M3 16.5h4"/><path d="M17 3v18"/><path d="M17 7.5h4"/><path d="M17 16.5h4"/></svg> <span data-i18n="color_strip.add">Add</span></h2>
             <button class="modal-close-btn" onclick="closeCSSEditorModal()" title="Close" data-i18n-aria-label="aria.close">&#x2715;</button>
         </div>
         <div class="modal-body">