Codebase review: stability, performance, usability, and i18n fixes

Stability:
- Fix race condition: set _is_running before create_task in target processors
- Await probe task after cancel in wled_target_processor
- Replace raw fetch() with fetchWithAuth() across devices, kc-targets, pattern-templates
- Add try/catch to showTestTemplateModal in streams.js
- Wrap blocking I/O in asyncio.to_thread (picture_targets, system restore)
- Fix dashboardStopAll to filter only running targets with ok guard

Performance:
- Vectorize fire effect spark loop with numpy in effect_stream
- Vectorize FFT band binning with cumulative sum in analysis.py
- Rewrite pixel_processor with vectorized numpy (accept ndarray or list)
- Add httpx.AsyncClient connection pooling with lock in wled_provider
- Optimize _send_pixels_http to avoid np.hstack allocation in wled_client
- Mutate chart arrays in-place in dashboard, perf-charts, targets
- Merge dashboard 2-batch fetch into single Promise.all
- Hoist frame_time outside loop in mapped_stream

Usability:
- Fix health check interval load/save in device settings
- Swap confirm modal button classes (No=secondary, Yes=danger)
- Add aria-modal to audio/value source editors, fix close button aria-labels
- Add modal footer close button to settings modal
- Add dedicated calibration LED count validation error keys

i18n:
- Replace ~50 hardcoded English strings with t() calls across 12 JS files
- Add 50 new keys to en.json, ru.json, zh.json
- Localize inline toasts in index.html with window.t fallback
- Add data-i18n to command palette footer
- Add localization policy to CLAUDE.md

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

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

@@ -242,19 +242,26 @@ function restoreCaptureDuration() {
 }
 
 export async function showTestTemplateModal(templateId) {
-    const templates = await fetchWithAuth('/capture-templates').then(r => r.json());
-    const template = templates.templates.find(t => t.id === templateId);
+    try {
+        const resp = await fetchWithAuth('/capture-templates');
+        if (!resp.ok) throw new Error(`HTTP ${resp.status}`);
+        const data = await resp.json();
+        const template = (data.templates || []).find(tp => tp.id === templateId);
 
-    if (!template) {
+        if (!template) {
+            showToast(t('templates.error.load'), 'error');
+            return;
+        }
+
+        window.currentTestingTemplate = template;
+        await loadDisplaysForTest();
+        restoreCaptureDuration();
+
+        testTemplateModal.open();
+    } catch (error) {
+        if (error.isAuth) return;
         showToast(t('templates.error.load'), 'error');
-        return;
     }
-
-    window.currentTestingTemplate = template;
-    await loadDisplaysForTest();
-    restoreCaptureDuration();
-
-    testTemplateModal.open();
 }
 
 export function closeTestTemplateModal() {
@@ -871,7 +878,7 @@ export async function cloneAudioTemplate(templateId) {
     } catch (error) {
         if (error.isAuth) return;
         console.error('Failed to clone audio template:', error);
-        showToast('Failed to clone audio template', 'error');
+        showToast(t('audio_template.error.clone_failed'), 'error');
     }
 }
 
@@ -2213,7 +2220,7 @@ export async function cloneStream(streamId) {
     } catch (error) {
         if (error.isAuth) return;
         console.error('Failed to clone stream:', error);
-        showToast('Failed to clone picture source', 'error');
+        showToast(t('stream.error.clone_picture_failed'), 'error');
     }
 }
 
@@ -2226,7 +2233,7 @@ export async function cloneCaptureTemplate(templateId) {
     } catch (error) {
         if (error.isAuth) return;
         console.error('Failed to clone capture template:', error);
-        showToast('Failed to clone capture template', 'error');
+        showToast(t('stream.error.clone_capture_failed'), 'error');
     }
 }
 
@@ -2239,7 +2246,7 @@ export async function clonePPTemplate(templateId) {
     } catch (error) {
         if (error.isAuth) return;
         console.error('Failed to clone PP template:', error);
-        showToast('Failed to clone postprocessing template', 'error');
+        showToast(t('stream.error.clone_pp_failed'), 'error');
     }
 }