Add composable filter templates, skip keepalive for serial devices

Filter Template meta-filter: reference existing PP templates inside others
for composable, DRY filter chains. Filters are recursively expanded at
pipeline build time with cycle detection. New `select` option type with
dynamic choices populated by the API.

Keepalive optimization: serial devices (Adalight, AmbiLED) don't need
keepalive — they hold last frame indefinitely. Check `standby_required`
capability at processor start, skip keepalive sends for serial targets,
and hide keepalive metrics in the UI. Rename "Standby Interval" to
"Keep Alive Interval" throughout the frontend.

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

server/src/wled_controller/static/js/features/streams.js

@@ -1175,6 +1175,22 @@ export function renderModalFilterList() {
                                 onchange="updateFilterOption(${index}, '${opt.key}', this.checked)">
                         </label>
                     </div>`;
+                } else if (opt.type === 'select' && Array.isArray(opt.choices)) {
+                    // Exclude the template being edited from filter_template choices (prevent self-reference)
+                    const editingId = document.getElementById('pp-template-id')?.value || '';
+                    const filteredChoices = (fi.filter_id === 'filter_template' && opt.key === 'template_id' && editingId)
+                        ? opt.choices.filter(c => c.value !== editingId)
+                        : opt.choices;
+                    const options = filteredChoices.map(c =>
+                        `<option value="${escapeHtml(c.value)}"${c.value === currentVal ? ' selected' : ''}>${escapeHtml(c.label)}</option>`
+                    ).join('');
+                    html += `<div class="pp-filter-option">
+                        <label for="${inputId}"><span>${escapeHtml(opt.label)}:</span></label>
+                        <select id="${inputId}"
+                            onchange="updateFilterOption(${index}, '${opt.key}', this.value)">
+                            ${options}
+                        </select>
+                    </div>`;
                 } else {
                     html += `<div class="pp-filter-option">
                         <label for="${inputId}">
@@ -1245,6 +1261,8 @@ export function updateFilterOption(filterIndex, optionKey, value) {
             const optDef = filterDef.options_schema.find(o => o.key === optionKey);
             if (optDef && optDef.type === 'bool') {
                 fi.options[optionKey] = !!value;
+            } else if (optDef && optDef.type === 'select') {
+                fi.options[optionKey] = String(value);
             } else if (optDef && optDef.type === 'int') {
                 fi.options[optionKey] = parseInt(value);
             } else {