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/ThemeToggle.svelte.spec.ts

@@ -0,0 +1,45 @@
+import { afterEach, beforeEach, describe, expect, it } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import { page } from 'vitest/browser';
+import ThemeToggle from './ThemeToggle.svelte';
+
+afterEach(() => {
+	cleanup();
+	localStorage.removeItem('theme');
+});
+
+describe('ThemeToggle — label derivation (light mode)', () => {
+	beforeEach(() => {
+		localStorage.setItem('theme', 'light');
+	});
+
+	it('aria-label invites switching to dark mode when theme is light', async () => {
+		render(ThemeToggle);
+		const btn = await page.getByRole('button').element();
+		expect(btn.getAttribute('aria-label')).toBe('Zu dunklem Design wechseln');
+	});
+
+	it('title equals aria-label in light mode', async () => {
+		render(ThemeToggle);
+		const btn = await page.getByRole('button').element();
+		expect(btn.getAttribute('title')).toBe(btn.getAttribute('aria-label'));
+	});
+});
+
+describe('ThemeToggle — label derivation (dark mode)', () => {
+	beforeEach(() => {
+		localStorage.setItem('theme', 'dark');
+	});
+
+	it('aria-label invites switching to light mode when theme is dark', async () => {
+		render(ThemeToggle);
+		const btn = await page.getByRole('button').element();
+		expect(btn.getAttribute('aria-label')).toBe('Zu hellem Design wechseln');
+	});
+
+	it('title equals aria-label in dark mode', async () => {
+		render(ThemeToggle);
+		const btn = await page.getByRole('button').element();
+		expect(btn.getAttribute('title')).toBe(btn.getAttribute('aria-label'));
+	});
+});