Add configurable time increment toggle for event creation

- Add toggle button (15/30 minutes) in calendar header next to navigation arrows
- Implement circular frosted styling consistent with nav buttons
- Add configurable snapping for drag-to-create events in week view
- Persist time increment setting across browser sessions using localStorage
- Update snap function to accept configurable increment instead of hardcoded 15 minutes

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

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

styles.css

@@ -410,12 +410,38 @@ body {
     transform: translateX(-50%);
 }
 
+.header-left {
+    display: flex;
+    align-items: center;
+    gap: 8px;
+}
+
 .header-right {
     display: flex;
     align-items: center;
     gap: 0.5rem;
 }
 
+.time-increment-button {
+    background: rgba(255,255,255,0.2);
+    border: none;
+    color: white;
+    font-size: 14px;
+    font-weight: bold;
+    width: 40px;
+    height: 40px;
+    border-radius: 50%;
+    cursor: pointer;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    transition: background-color 0.2s;
+}
+
+.time-increment-button:hover {
+    background: rgba(255,255,255,0.3);
+}
+
 .nav-button {
     background: rgba(255,255,255,0.2);
     border: none;