Add pluggable postprocessing filter system with collapsible UI

Replace hardcoded gamma/saturation/brightness fields with a flexible
filter pipeline architecture. Templates now contain an ordered list of
filter instances, each with its own options schema. Filters operate on
full images before border extraction.

- Add filter framework: base class, registry, image pool, filter instance
- Implement 6 built-in filters: brightness, saturation, gamma, downscaler, pixelate, auto crop
- Move smoothing from PP templates to device stream settings (temporal, not spatial)
- Add GET /api/v1/filters endpoint for available filter types
- Dynamic filter UI in template modal with add/remove/reorder/collapse
- Replace camera icon with display icon for screen capture streams
- Legacy migration: existing templates auto-convert flat fields to filter list

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/static/style.css

@@ -1926,6 +1926,163 @@ input:-webkit-autofill:focus {
     font-size: 13px;
 }
 
+/* PP Filter List in Template Modal */
+.pp-filter-list {
+    display: flex;
+    flex-direction: column;
+    gap: 8px;
+    margin-bottom: 12px;
+}
+
+.pp-filter-empty {
+    color: var(--text-secondary);
+    font-size: 13px;
+    text-align: center;
+    padding: 16px;
+    border: 1px dashed var(--border-color);
+    border-radius: 8px;
+}
+
+.pp-filter-card {
+    border: 1px solid var(--border-color);
+    border-radius: 8px;
+    background: var(--bg-secondary);
+    padding: 10px 12px;
+}
+
+.pp-filter-card-header {
+    display: flex;
+    align-items: center;
+    gap: 6px;
+    cursor: pointer;
+    user-select: none;
+}
+
+.pp-filter-card.expanded .pp-filter-card-header {
+    margin-bottom: 8px;
+}
+
+.pp-filter-card-chevron {
+    font-size: 10px;
+    color: var(--text-secondary);
+    flex-shrink: 0;
+    width: 12px;
+}
+
+.pp-filter-card-name {
+    font-weight: 600;
+    font-size: 14px;
+    color: var(--text-primary);
+}
+
+.pp-filter-card-summary {
+    color: var(--text-secondary);
+    font-size: 12px;
+    margin-right: 8px;
+    white-space: nowrap;
+    overflow: hidden;
+    text-overflow: ellipsis;
+}
+
+.pp-filter-card-actions {
+    display: flex;
+    gap: 4px;
+    flex-shrink: 0;
+    margin-left: auto;
+}
+
+.btn-filter-action {
+    background: none;
+    border: 1px solid var(--border-color);
+    border-radius: 4px;
+    color: var(--text-secondary);
+    cursor: pointer;
+    width: 26px;
+    height: 26px;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    font-size: 11px;
+    padding: 0;
+}
+
+.btn-filter-action:hover:not(:disabled) {
+    background: var(--border-color);
+    color: var(--text-primary);
+}
+
+.btn-filter-action:disabled {
+    opacity: 0.3;
+    cursor: not-allowed;
+}
+
+.btn-filter-remove:hover:not(:disabled) {
+    background: rgba(239, 68, 68, 0.15);
+    border-color: rgba(239, 68, 68, 0.4);
+    color: #ef4444;
+}
+
+.pp-filter-card-options {
+    display: flex;
+    flex-direction: column;
+    gap: 4px;
+}
+
+.pp-filter-option {
+    display: flex;
+    flex-direction: column;
+    gap: 2px;
+}
+
+.pp-filter-option label {
+    display: flex;
+    justify-content: space-between;
+    font-size: 12px;
+    color: var(--text-secondary);
+}
+
+.pp-filter-option input[type="range"] {
+    width: 100%;
+}
+
+.pp-add-filter-row {
+    display: flex;
+    gap: 8px;
+    align-items: center;
+    margin-bottom: 4px;
+}
+
+.pp-add-filter-select {
+    flex: 1;
+    padding: 6px 10px;
+    border: 1px solid var(--border-color);
+    border-radius: 6px;
+    background: var(--card-bg);
+    color: var(--text-primary);
+    font-size: 13px;
+}
+
+.pp-add-filter-btn {
+    width: 34px;
+    height: 34px;
+    flex-shrink: 0;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    border: 1px solid var(--border-color);
+    border-radius: 6px;
+    background: var(--card-bg);
+    color: var(--text-primary);
+    font-size: 20px;
+    cursor: pointer;
+    padding: 0;
+    line-height: 1;
+}
+
+.pp-add-filter-btn:hover {
+    background: var(--border-color);
+}
+
 /* Template Test Section */
 .template-test-section {
     background: var(--bg-secondary);