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/css/components.css

@@ -689,6 +689,25 @@ textarea:focus-visible {
     pointer-events: auto;
 }
 
+.icon-select-search {
+    width: calc(100% - 12px);
+    padding: 8px 12px;
+    margin: 6px 6px 0;
+    border: 1px solid var(--border-color);
+    border-radius: var(--radius);
+    background: var(--bg-secondary);
+    color: var(--text-color);
+    font-size: 0.9rem;
+    outline: none;
+    box-sizing: border-box;
+}
+.icon-select-search::placeholder {
+    color: var(--text-secondary);
+}
+.icon-select-search:focus {
+    border-color: var(--primary-color);
+}
+
 .icon-select-grid {
     display: grid;
     grid-auto-rows: 1fr;
@@ -820,6 +839,7 @@ textarea:focus-visible {
     /* Override inline columns — use responsive auto-fill */
     grid-template-columns: repeat(auto-fill, minmax(150px, 1fr)) !important;
 }
+.icon-select-popup .icon-select-cell.disabled,
 .type-picker-dialog .icon-select-cell.disabled {
     opacity: 0.25;
     pointer-events: none;