feat(nav): add cursor-pointer and tooltip to notification bell

Extract bellLabel as $derived to DRY up aria-label and title.
Add cursor-pointer globally to button via @layer base so Tailwind
preflight reset doesn't override the browser default.

Closes #344

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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);