fix: improve SPC dashboard UX — hover toolbar, visible report errors

- Change Plotly modebar to hover-only mode to avoid overlapping chart content
- Remove redundant Plotly titles (container headers already provide them)
- Add Content-Type check and inline error display for report downloads
- Fix setup login validation and seed idempotency for existing data

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

server/routers/setup.py

@@ -268,6 +268,14 @@ async def seed_demo_data(body: PasswordBody):
 
             created_users: list[User] = []
             for u in users_data:
+                # Skip users that already exist
+                existing = await session.execute(
+                    select(User).where(User.username == u["username"])
+                )
+                existing_user = existing.scalar_one_or_none()
+                if existing_user:
+                    created_users.append(existing_user)
+                    continue
                 user = User(
                     username=u["username"],
                     password_hash=hash_password(u["password"]),
@@ -286,6 +294,19 @@ async def seed_demo_data(body: PasswordBody):
             admin_user = created_users[0]
 
             # ---- Recipe -------------------------------------------------------
+            # Skip if recipe already exists
+            existing_recipe = await session.execute(
+                select(Recipe).where(Recipe.code == "DEMO-001")
+            )
+            if existing_recipe.scalar_one_or_none():
+                return {
+                    "status": "ok",
+                    "message": "Demo data already exists, skipped",
+                    "users_created": [u["username"] for u in users_data],
+                    "recipe_created": "DEMO-001",
+                    "measurements_created": 0,
+                }
+
             recipe = Recipe(
                 code="DEMO-001",
                 name="Demo Measurement Recipe",