HRV balanced dots, dashed gap lines, dashboard widgets + drag-to-edit layout

- Green dots for balanced HRV (joining orange unbalanced / red low) on the trend
- Trend charts bridge data gaps with a dashed line instead of a blank break
- Dashboard: drop Health today; add VO2 max, small sleep, and HRV status widgets
- Dashboard: editable widget grid (react-grid-layout) with drag/resize, saved per-user

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/api/profile.py

@@ -35,11 +35,16 @@ class ProfileOut(BaseModel):
     goal_weight_kg: Optional[float]
     estimated_max_hr: Optional[int]
     is_admin: bool
+    dashboard_layout: Optional[list] = None
 
     class Config:
         from_attributes = True
 
 
+class DashboardLayoutIn(BaseModel):
+    layout: Optional[list] = None   # react-grid-layout array of {i,x,y,w,h}
+
+
 def _estimated_max_hr(user: User) -> Optional[int]:
     if user.birth_year:
         return 220 - (datetime.now().year - user.birth_year)
@@ -53,6 +58,18 @@ async def get_profile(current_user: User = Depends(get_current_user)):
             "estimated_max_hr": _estimated_max_hr(current_user)}
 
 
+@router.put("/dashboard-layout")
+async def save_dashboard_layout(
+    body: DashboardLayoutIn,
+    db: AsyncSession = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+):
+    """Persist the user's customised dashboard widget layout."""
+    current_user.dashboard_layout = body.layout
+    await db.commit()
+    return {"status": "ok"}
+
+
 @router.patch("/", response_model=ProfileOut)
 async def update_profile(
     body: ProfileUpdate,