feat: per-account All Mail folder selection

Replaces the global allMailFolderIds (string[] | null) with a per-account
Record<accountId, string[]>, so each account chooses which of its own folders
the "All Mail" view merges. A missing entry = "not configured" (defaults to
every no-role folder); an explicit [] = "no folders".

- settings-store: type/default -> Record (default {}); persist version 4 -> 5,
  migration drops the legacy global list (the active account isn't known at
  migrate time); onRehydrate + importSettings coerce/ignore any non-record
  (legacy global string[] | null) shape. isPlainRecord() guard.
- email-store.resolveAllMailJmapIds: reads the entry for the account the view is
  scoped to (viewingAccountId ?? activeAccountId); undefined -> all no-role,
  [] -> none.
- layout-settings: read/write the active account's entry; when more than one
  account is logged in, an italic hint names the account the selection applies
  to (settings.appearance.all_mail.account_hint, 19 locales; de/ro translated).
- Test: stores/__tests__/settings-store-all-mail.test.ts (per-account
  independence, explicit-empty vs not-configured, importSettings legacy guard).

stores/__tests__/settings-store-all-mail.test.ts

@@ -0,0 +1,55 @@
+import { describe, it, expect, beforeEach } from 'vitest';
+import { useSettingsStore } from '../settings-store';
+
+describe('settings-store per-account allMailFolderIds', () => {
+  beforeEach(() => {
+    useSettingsStore.setState({ allMailFolderIds: {} });
+  });
+
+  it('defaults to an empty record (every account "not configured")', () => {
+    expect(useSettingsStore.getState().allMailFolderIds).toEqual({});
+  });
+
+  it('keeps each account selection independent', () => {
+    useSettingsStore.setState({
+      allMailFolderIds: { 'acct-1': ['inbox', 'archive'], 'acct-2': ['sent'] },
+    });
+    const map = useSettingsStore.getState().allMailFolderIds;
+    expect(map['acct-1']).toEqual(['inbox', 'archive']);
+    expect(map['acct-2']).toEqual(['sent']);
+    // A third account remains unconfigured (no entry).
+    expect(map['acct-3']).toBeUndefined();
+  });
+
+  it('distinguishes explicit-empty ([] = no folders) from not-configured (undefined)', () => {
+    useSettingsStore.setState({ allMailFolderIds: { 'acct-1': [] } });
+    const map = useSettingsStore.getState().allMailFolderIds;
+    expect(map['acct-1']).toEqual([]);          // explicit "no folders"
+    expect(map['acct-2']).toBeUndefined();      // never configured
+  });
+
+  describe('importSettings legacy-shape guard', () => {
+    it('ignores a legacy global array shape', () => {
+      useSettingsStore.setState({ allMailFolderIds: { 'acct-1': ['inbox'] } });
+      const ok = useSettingsStore.getState().importSettings(
+        JSON.stringify({ allMailFolderIds: ['inbox', 'sent'] }),
+      );
+      expect(ok).toBe(true);
+      // unchanged - the array shape was rejected
+      expect(useSettingsStore.getState().allMailFolderIds).toEqual({ 'acct-1': ['inbox'] });
+    });
+
+    it('ignores a null legacy value', () => {
+      useSettingsStore.setState({ allMailFolderIds: { 'acct-1': ['inbox'] } });
+      useSettingsStore.getState().importSettings(JSON.stringify({ allMailFolderIds: null }));
+      expect(useSettingsStore.getState().allMailFolderIds).toEqual({ 'acct-1': ['inbox'] });
+    });
+
+    it('accepts a proper per-account record', () => {
+      useSettingsStore.getState().importSettings(
+        JSON.stringify({ allMailFolderIds: { 'acct-9': ['inbox', 'spam'] } }),
+      );
+      expect(useSettingsStore.getState().allMailFolderIds).toEqual({ 'acct-9': ['inbox', 'spam'] });
+    });
+  });
+});