feat(nav): add tooltip and cursor:pointer to notification bell, fix ThemeToggle i18n (#344) (#351)

Closes #344

## What was implemented

### Commit 1 — `feat(nav): add cursor-pointer and tooltip to notification bell`
- Extracted `bellLabel` as `$derived` in `NotificationBell.svelte` — eliminates the duplicated inline ternary and keeps tooltip/label in sync reactively
- Added `title={bellLabel}` to the bell `<button>` — native tooltip mirrors `aria-label` in both zero and non-zero unread states
- Added `cursor-pointer` to the bell button's class list
- Added global `button { cursor: pointer; }` rule in `@layer base` of `layout.css` — prevents future regressions (global scope per Decision Queue)
- Added 3 component tests in `NotificationBell.svelte.spec.ts`: cursor-pointer class present, title equals aria-label when unread=0, title equals aria-label when unread=3

### Commit 2 — `fix(nav): replace hardcoded ThemeToggle title with Paraglide i18n keys`
- Added `theme_toggle_to_light` / `theme_toggle_to_dark` keys to `de/en/es` messages
- Extracted `themeLabel` as `$derived` in `ThemeToggle.svelte` and bound both `aria-label` and `title` to it
- Fixes the pre-existing hardcoded English strings (`'light mode'` / `'dark mode'`) per Decision Queue resolution

Touch target size was descoped per the Decision Queue.

## Decision Queue resolutions (from issue #344)
- **cursor-pointer scope**: global via `@layer base` ✅
- **ThemeToggle scope**: fixed in this issue ✅
- **Touch target**: descoped ✅

## Test results
All 5 `NotificationBell` tests pass.

Co-authored-by: Marcel <marcel@familienarchiv>
Reviewed-on: http://heim-nas:3005/marcel/familienarchiv/pulls/351

frontend/src/lib/components/NotificationBell.svelte.spec.ts

@@ -55,6 +55,34 @@ async function openDropdownAndClickFirstNotification() {
 	notifButton.click();
 }
 
+describe('NotificationBell — cursor and tooltip', () => {
+	it('bell button has cursor-pointer class', async () => {
+		render(NotificationBell);
+		const btn = document.querySelector<HTMLButtonElement>('button[aria-haspopup="true"]')!;
+		expect(btn.classList.contains('cursor-pointer')).toBe(true);
+	});
+
+	it('bell button title equals aria-label when unreadCount is 0', async () => {
+		mockNotificationList.value = [];
+		render(NotificationBell);
+		const btn = document.querySelector<HTMLButtonElement>('button[aria-haspopup="true"]')!;
+		expect(btn.getAttribute('title')).toBe('Benachrichtigungen');
+		expect(btn.getAttribute('aria-label')).toBe(btn.getAttribute('title'));
+	});
+
+	it('bell button title equals aria-label when unreadCount is 3', async () => {
+		mockNotificationList.value = [
+			makeNotification({ id: 'n1' }),
+			makeNotification({ id: 'n2' }),
+			makeNotification({ id: 'n3' })
+		];
+		render(NotificationBell);
+		const btn = document.querySelector<HTMLButtonElement>('button[aria-haspopup="true"]')!;
+		expect(btn.getAttribute('title')).toBe('3 ungelesene Benachrichtigungen');
+		expect(btn.getAttribute('aria-label')).toBe(btn.getAttribute('title'));
+	});
+});
+
 describe('NotificationBell', () => {
 	it('handleMarkRead navigates to URL including annotationId when notification has annotationId', async () => {
 		mockNotificationList.value = [makeNotification({ annotationId: 'annot-1' })];