fix: persist active account across reload, remount views on switch

Aligns with upstream stalwartlabs/webui@189e270 (v1.0.7, "fixes #17"),
reviewed after they independently landed a fix for the same issue:

- authStore now persists activeAccountId (sessionStorage) and
  setSession preserves it across a session refresh instead of always
  resetting to primaryAccountId, so a hard reload keeps you on the
  group account you had selected instead of bouncing back to your own.
- switchAccount now clears cacheStore (displayNames/objectLists) when
  actually changing account, since those were resolved against the
  previous account and would otherwise show stale labels.
- The ErrorBoundary wrapping MainContent is now keyed on
  activeAccountId, forcing a full remount of every view on switch.
  This is more robust than gating individual components' fetch
  effects on activeAccountId (our earlier fix in DynamicList.tsx,
  kept as-is — harmless now, but no longer load-bearing on its own)
  since it covers every current and future view type, not just lists.

Verified against a live instance: switching to a group account updates
the JMAP accountId immediately (no tab switch needed), and a full page
reload keeps the group account active instead of resetting to admin.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/stores/authStore.test.ts

@@ -6,6 +6,7 @@
 
 import { describe, it, expect, beforeEach, vi } from 'vitest';
 import { useAuthStore } from './authStore';
+import { useCacheStore } from './cacheStore';
 
 const initialState = {
   accessToken: null,
@@ -21,6 +22,7 @@ const initialState = {
 describe('authStore', () => {
   beforeEach(() => {
     useAuthStore.setState(initialState);
+    useCacheStore.setState({ displayNames: {}, objectLists: {} });
     vi.restoreAllMocks();
   });
 
@@ -124,6 +126,26 @@ describe('authStore', () => {
 
       expect(useAuthStore.getState().activeAccountId).toBe('acc-1');
     });
+
+    it('preserves the previously active account across a reload if still valid', () => {
+      useAuthStore.setState({ activeAccountId: 'acc-2' });
+      const accounts = {
+        'acc-1': { name: 'Personal', isPersonal: true },
+        'acc-2': { name: 'Shared', isPersonal: false },
+      };
+
+      useAuthStore.getState().setSession(accounts, 'acc-1', 'https://api');
+
+      expect(useAuthStore.getState().activeAccountId).toBe('acc-2');
+    });
+
+    it('falls back to primaryAccountId if the previously active account is gone', () => {
+      useAuthStore.setState({ activeAccountId: 'acc-removed' });
+
+      useAuthStore.getState().setSession({ 'acc-1': { name: 'A', isPersonal: true } }, 'acc-1', 'https://api');
+
+      expect(useAuthStore.getState().activeAccountId).toBe('acc-1');
+    });
   });
 
   describe('switchAccount', () => {
@@ -149,6 +171,33 @@ describe('authStore', () => {
       useAuthStore.getState().switchAccount('nonexistent');
       expect(useAuthStore.getState().activeAccountId).toBe('a1');
     });
+
+    it('clears the cache store when switching to a different account', () => {
+      useAuthStore.setState({
+        accounts: {
+          a1: { name: 'A1', isPersonal: true },
+          a2: { name: 'A2', isPersonal: false },
+        },
+        activeAccountId: 'a1',
+      });
+      useCacheStore.getState().setDisplayNames('x:Domain', { d1: 'example.org' });
+
+      useAuthStore.getState().switchAccount('a2');
+
+      expect(useCacheStore.getState().displayNames).toEqual({});
+    });
+
+    it('does not clear the cache store when "switching" to the already-active account', () => {
+      useAuthStore.setState({
+        accounts: { a1: { name: 'A1', isPersonal: true } },
+        activeAccountId: 'a1',
+      });
+      useCacheStore.getState().setDisplayNames('x:Domain', { d1: 'example.org' });
+
+      useAuthStore.getState().switchAccount('a1');
+
+      expect(useCacheStore.getState().displayNames).toEqual({ 'x:Domain': { d1: 'example.org' } });
+    });
   });
 
   describe('logout', () => {