feat: implement skeleton loading screens for all views (Phase 3.5)

Created comprehensive skeleton components providing visual feedback during
data loading. This improves perceived performance and provides a better
user experience with consistent loading states across all views.

Changes:
- NEW: src/components/LoadingSkeletons.tsx (+105 lines)
  - SealedSecretListSkeleton - 5 placeholder rows
  - SealedSecretDetailSkeleton - title + sections + actions
  - SealingKeysListSkeleton - 2 certificate placeholders
  - CertificateInfoSkeleton - metadata lines
  - ControllerHealthSkeleton - chip + info layout
  - All use wave animation and realistic layouts

- UPDATED: SealedSecretList.tsx
  - Use loading state from useList() hook
  - Show skeleton during data fetch
  - Smooth transition to real data

- UPDATED: SealedSecretDetail.tsx
  - Replace Headlamp Loader with custom skeleton
  - Better layout matching
  - No layout shift

- UPDATED: SealingKeysView.tsx
  - Add loading state detection
  - Show skeleton for certificates
  - Professional loading UX

- UPDATED: ControllerStatus.tsx
  - Replace CircularProgress with skeleton
  - Match chip + info layout
  - Consistent with other components

Benefits:
- Improved perceived performance
- Reduced layout shift (skeletons match real components)
- Consistent loading experience (wave animation)
- Better user feedback during data loading

Build: 356.44 kB (98.01 kB gzipped) - +1.52 kB (+0.4%)
Time: 4.78s

Progress: 11/14 phases complete (79%)

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

headlamp-sealed-secrets/src/components/ControllerStatus.tsx

@@ -6,9 +6,10 @@
  */
 
 import { CheckCircle, Error as ErrorIcon, Warning } from '@mui/icons-material';
-import { Box, Chip, CircularProgress, Tooltip, Typography } from '@mui/material';
+import { Box, Chip, Tooltip, Typography } from '@mui/material';
 import React from 'react';
 import { useControllerHealth } from '../hooks/useControllerHealth';
+import { ControllerHealthSkeleton } from './LoadingSkeletons';
 
 interface ControllerStatusProps {
   /** Whether to auto-refresh the status */
@@ -29,15 +30,9 @@ export function ControllerStatus({
 }: ControllerStatusProps) {
   const { health: status, loading } = useControllerHealth(autoRefresh, refreshIntervalMs);
 
+  // Show skeleton while loading
   if (loading || !status) {
-    return (
-      <Box sx={{ display: 'flex', alignItems: 'center', gap: 1 }}>
-        <CircularProgress size={16} />
-        <Typography variant="body2" color="text.secondary">
-          Checking controller...
-        </Typography>
-      </Box>
-    );
+    return <ControllerHealthSkeleton />;
   }
 
   // Build status message and icon