feat(list): click sender avatar to select message/thread (Thunderbird-style)

Wrap the message-list avatar in a SelectableAvatar control: clicking the
avatar toggles the row into the current selection instead of opening it,
matching Thunderbird's correspondent-avatar selection affordance. A check
overlay appears on hover (hinting it is clickable) and stays while selected.

- email-list-item + thread single-email: toggle that message's id
- thread header: toggle the whole thread (reuses existing thread-select logic)
- focused-mail and extra-compact layouts render no avatar, so unaffected

components/email/__tests__/selectable-avatar.test.tsx

@@ -0,0 +1,39 @@
+import { describe, it, expect, vi } from 'vitest';
+import { render, screen, fireEvent } from '@testing-library/react';
+import { SelectableAvatar } from '../selectable-avatar';
+
+// Isolate from the real Avatar (image fetching, libravatar hashing) — we only
+// care about the selection wrapper behaviour here.
+vi.mock('@/components/ui/avatar', () => ({
+  Avatar: (props: { name?: string }) => <span data-testid="avatar">{props.name}</span>,
+}));
+
+describe('SelectableAvatar', () => {
+  it('renders the wrapped avatar', () => {
+    render(<SelectableAvatar name="Marta" checked={false} onToggle={() => {}} selectLabel="Select" />);
+    expect(screen.getByTestId('avatar')).toHaveTextContent('Marta');
+  });
+
+  it('fires onToggle and stops propagation when the avatar is clicked', () => {
+    const onToggle = vi.fn();
+    const onRowClick = vi.fn();
+    render(
+      <div onClick={onRowClick}>
+        <SelectableAvatar name="Marta" checked={false} onToggle={onToggle} selectLabel="Select" />
+      </div>,
+    );
+    fireEvent.click(screen.getByRole('checkbox'));
+    expect(onToggle).toHaveBeenCalledTimes(1);
+    // Clicking the avatar must not bubble up to open/select the row.
+    expect(onRowClick).not.toHaveBeenCalled();
+  });
+
+  it('reflects the checked state via aria-checked', () => {
+    const { rerender } = render(
+      <SelectableAvatar name="Marta" checked={false} onToggle={() => {}} selectLabel="Select" />,
+    );
+    expect(screen.getByRole('checkbox')).toHaveAttribute('aria-checked', 'false');
+    rerender(<SelectableAvatar name="Marta" checked onToggle={() => {}} selectLabel="Select" />);
+    expect(screen.getByRole('checkbox')).toHaveAttribute('aria-checked', 'true');
+  });
+});