fix: Phase 2 QA — all 25 remaining HIGH/MEDIUM/LOW issues

HIGH fixes (7):
- H1: VNCdirectory admin i18n — 30+ translation keys added
- H2: handleSave try/catch with error toast
- H3: Free/busy accountId scoping
- H4: cancelEventBookings filter by eventId
- H5: Resource picker static apiFetch import
- H6: Sharing-store toast messages via lastMessage state
- H7: roleLabel for all resource types

MEDIUM fixes (11):
- M1: identitySignatureMap cleanup on delete
- M2: Now-line relative positioning
- M3: Radial menu disabled item keyboard nav
- M4: Radial menu stable event listener via refs
- M5: cancelBooking error on missing booking
- M6: PasswordRow isMasked state flag
- M7: Extract shared rights into lib/sharing-rights.ts
- M8: VNCtalk client server-side guard
- M9: Collabora configManager instead of process.env
- M10: CONFIG_ENV_MAP VNCdirectory fields
- M11: SENSITIVE_CONFIG_KEYS field name unification

LOW fixes (7):
- L1-L3: Unused imports removed
- L4: aria-labels on close, clear, search, spinner
- L5-L7: Comments for intentional patterns, null guard

stores/signature-store.ts

@@ -61,11 +61,28 @@ export const useSignatureStore = create<SignatureState>()(
       },
 
       deleteSignature: (id) => {
-        set((state) => ({
-          signatures: state.signatures.filter((s) => s.id !== id),
-          defaultSignatureId: state.defaultSignatureId === id ? null : state.defaultSignatureId,
-          replySignatureId: state.replySignatureId === id ? null : state.replySignatureId,
-        }));
+        set((state) => {
+          const nextMap = { ...state.identitySignatureMap };
+          for (const identityId of Object.keys(nextMap)) {
+            const entry = nextMap[identityId];
+            if (entry.defaultId === id || entry.replyId === id) {
+              const updated = { ...entry };
+              if (updated.defaultId === id) delete updated.defaultId;
+              if (updated.replyId === id) delete updated.replyId;
+              if (Object.keys(updated).length === 0) {
+                delete nextMap[identityId];
+              } else {
+                nextMap[identityId] = updated;
+              }
+            }
+          }
+          return {
+            signatures: state.signatures.filter((s) => s.id !== id),
+            defaultSignatureId: state.defaultSignatureId === id ? null : state.defaultSignatureId,
+            replySignatureId: state.replySignatureId === id ? null : state.replySignatureId,
+            identitySignatureMap: nextMap,
+          };
+        });
       },
 
       duplicateSignature: (id) => {