feat(calendar): improve settings page UI and add default calendar selection

- Add setAsDefault function to calendars store
- Redesign calendar edit form with better input styling
- Use shared SettingsSection and SettingsCard components
- Add sticky pill headers with icons for all sections
- Increase spacing between sections (2rem gap)
- Add plus icon to "Neuer Kalender" button
- Style improvements for consistency with GlobalSettingsSection

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/calendar/apps/web/src/lib/components/calendar/DateStrip.svelte

@@ -488,11 +488,23 @@
 	}
 
 	.day-item.weekend {
-		background: hsl(var(--color-muted) / 0.5);
+		background: transparent;
+		border: 1px solid hsl(var(--color-border));
 	}
 
 	.day-item.weekend:hover {
-		background: hsl(var(--color-muted) / 0.8);
+		background: hsl(var(--color-muted) / 0.3);
+		border-color: hsl(var(--color-muted-foreground) / 0.5);
+	}
+
+	/* Weekend + in-range combination */
+	.day-item.weekend.in-range {
+		background: hsl(var(--color-primary) / 0.15);
+		border: 1px solid hsl(var(--color-primary) / 0.4);
+	}
+
+	.day-item.weekend.in-range:hover {
+		background: hsl(var(--color-primary) / 0.25);
 	}
 
 	.day-item.selected {

apps/calendar/apps/web/src/routes/(app)/settings/+page.svelte

@@ -170,7 +170,18 @@
 		<SettingsCard>
 			<div class="p-5">
 				<div class="calendars-toolbar">
-					<button class="btn btn-primary btn-sm" onclick={() => (showNewCalendarForm = true)}>
+					<button
+						class="inline-flex items-center gap-2 px-4 py-2 text-sm font-medium rounded-lg transition-colors bg-[hsl(var(--primary))] text-[hsl(var(--primary-foreground))] hover:bg-[hsl(var(--primary))]/90"
+						onclick={() => (showNewCalendarForm = true)}
+					>
+						<svg class="w-4 h-4" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+							<path
+								stroke-linecap="round"
+								stroke-linejoin="round"
+								stroke-width="2"
+								d="M12 4v16m8-8H4"
+							/>
+						</svg>
 						Neuer Kalender
 					</button>
 				</div>
@@ -579,10 +590,13 @@
 		max-width: 600px;
 		margin: 0 auto;
 		padding-bottom: 2rem;
+		display: flex;
+		flex-direction: column;
+		gap: 2rem;
 	}
 
 	.page-header {
-		margin-bottom: 1.5rem;
+		margin-bottom: 0;
 	}
 
 	.page-header h1 {