Add Stargil Nails logo and favicon

- Replace emoji icons with Stargil Nails logo in header and loading spinner
- Add favicon.png to public directory
- Copy logo to public/assets for browser access
- Update vite.config.ts to serve public directory
- Add favicon link to HTML head section

src/server/rpc/availability.ts

@@ -41,18 +41,29 @@ const create = os
     })
   )
   .handler(async ({ input }) => {
-    await assertOwner(input.sessionId);
-    const id = randomUUID();
-    const slot: Availability = {
-      id,
-      date: input.date,
-      time: input.time,
-      durationMinutes: input.durationMinutes,
-      status: "free",
-      createdAt: new Date().toISOString(),
-    };
-    await kv.setItem(id, slot);
-    return slot;
+    try {
+      await assertOwner(input.sessionId);
+      // Prevent duplicate slot on same date+time
+      const existing = await kv.getAllItems();
+      const conflict = existing.some((s) => s.date === input.date && s.time === input.time);
+      if (conflict) {
+        throw new Error("Es existiert bereits ein Slot zu diesem Datum und dieser Uhrzeit.");
+      }
+      const id = randomUUID();
+      const slot: Availability = {
+        id,
+        date: input.date,
+        time: input.time,
+        durationMinutes: input.durationMinutes,
+        status: "free",
+        createdAt: new Date().toISOString(),
+      };
+      await kv.setItem(id, slot);
+      return slot;
+    } catch (err) {
+      console.error("availability.create error", err);
+      throw err;
+    }
   });
 
 const update = os