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/routes/layout.css

@@ -365,6 +365,11 @@
 		text-underline-offset: 4px;
 	}
 
+	/* Tailwind preflight resets cursor on *, overriding the browser default for buttons */
+	button {
+		cursor: pointer;
+	}
+
 	/* Fallback focus ring for any interactive element not styled with ring-focus-ring */
 	:focus-visible {
 		outline: 2px solid var(--c-focus-ring);