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

@@ -1,5 +1,6 @@
 <script lang="ts">
 import { onMount } from 'svelte';
+import { m } from '$lib/paraglide/messages.js';
 
 type Theme = 'light' | 'dark';
 
@@ -19,6 +20,10 @@ onMount(() => {
 	theme = resolveInitialTheme();
 });
 
+const themeLabel = $derived(
+	theme === 'dark' ? m.theme_toggle_to_light() : m.theme_toggle_to_dark()
+);
+
 function toggle() {
 	theme = theme === 'dark' ? 'light' : 'dark';
 	localStorage.setItem('theme', theme);
@@ -29,8 +34,8 @@ function toggle() {
 <button
 	type="button"
 	onclick={toggle}
-	aria-label={theme === 'dark' ? 'light mode' : 'dark mode'}
-	title={theme === 'dark' ? 'light mode' : 'dark mode'}
+	aria-label={themeLabel}
+	title={themeLabel}
 	class="rounded p-1.5 text-white/65 transition-colors hover:bg-white/10 hover:text-white focus:outline-none focus-visible:ring-2 focus-visible:ring-focus-ring"
 >
 	{#if theme === 'dark'}