fix: replace @mui/icons-material with @iconify/react (v0.2.3)

Material-UI icons were not provided as globals by Headlamp, causing
'undefined is not an object (evaluating Ct.createSvgIcon)' errors.

Headlamp provides @iconify/react as a global, so all icon imports have
been replaced with Iconify equivalents:
- ErrorOutline → mdi:alert-circle-outline
- ContentCopy → mdi:content-copy
- Visibility → mdi:eye
- VisibilityOff → mdi:eye-off
- CheckCircle → mdi:check-circle
- Error → mdi:alert-circle
- Warning → mdi:alert
- Add → mdi:plus
- Delete → mdi:delete

Changes:
- Replaced all @mui/icons-material imports with @iconify/react Icon component
- Updated 4 component files (ErrorBoundary, DecryptDialog, EncryptDialog, ControllerStatus)
- Bumped version to 0.2.3
- Bundle size reduced: 358.18 kB (98.04 kB gzipped)
- Checksum: SHA256:03787323abc9430a63433838253b2dd8296d237000acdfe4ce2507678b63125f

This should fix the plugin loading issue and make the sidebar entry appear.

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

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

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

@@ -6,7 +6,7 @@
  */
 
 import { K8s } from '@kinvolk/headlamp-plugin/lib';
-import { Add as AddIcon, Delete as DeleteIcon, Visibility, VisibilityOff } from '@mui/icons-material';
+import { Icon } from '@iconify/react';
 import {
   Box,
   Button,
@@ -222,7 +222,7 @@ export function EncryptDialog({ open, onClose }: EncryptDialogProps) {
                       aria-label={kv.showValue ? 'Hide password' : 'Show password'}
                       tabIndex={0}
                     >
-                      {kv.showValue ? <VisibilityOff /> : <Visibility />}
+                      {kv.showValue ? <Icon icon="mdi:eye-off" /> : <Icon icon="mdi:eye" />}
                     </IconButton>
                   ),
                 }}
@@ -234,13 +234,13 @@ export function EncryptDialog({ open, onClose }: EncryptDialogProps) {
                 aria-label={`Remove key-value pair ${index + 1}`}
                 title={keyValues.length === 1 ? 'At least one key-value pair is required' : `Remove key-value pair ${index + 1}`}
               >
-                <DeleteIcon />
+                <Icon icon="mdi:delete" />
               </IconButton>
             </Box>
           ))}
 
           <Button
-            startIcon={<AddIcon />}
+            startIcon={<Icon icon="mdi:plus" />}
             onClick={handleAddKeyValue}
             aria-label="Add another key-value pair"
           >