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/kc-targets.js

@@ -505,7 +505,7 @@ export async function showKCEditor(targetId = null, cloneData = null) {
         setTimeout(() => document.getElementById('kc-editor-name').focus(), 100);
     } catch (error) {
         console.error('Failed to open KC editor:', error);
-        showToast('Failed to open key colors editor', 'error');
+        showToast(t('kc_target.error.editor_open_failed'), 'error');
     }
 }
 
@@ -588,13 +588,13 @@ export async function saveKCEditor() {
 
 export async function cloneKCTarget(targetId) {
     try {
-        const resp = await fetch(`${API_BASE}/picture-targets/${targetId}`, { headers: getHeaders() });
+        const resp = await fetchWithAuth(`/picture-targets/${targetId}`);
         if (!resp.ok) throw new Error('Failed to load target');
         const target = await resp.json();
         showKCEditor(null, target);
     } catch (error) {
-        console.error('Failed to clone KC target:', error);
-        showToast('Failed to clone key colors target', 'error');
+        if (error.isAuth) return;
+        showToast(t('kc_target.error.clone_failed'), 'error');
     }
 }
 
@@ -613,11 +613,11 @@ export async function deleteKCTarget(targetId) {
             if (typeof window.loadTargetsTab === 'function') window.loadTargetsTab();
         } else {
             const error = await response.json();
-            showToast(`Failed to delete: ${error.detail}`, 'error');
+            showToast(t('kc_target.error.delete_failed'), 'error');
         }
     } catch (error) {
         if (error.isAuth) return;
-        showToast('Failed to delete key colors target', 'error');
+        showToast(t('kc_target.error.delete_failed'), 'error');
     }
 }