Add bottle size, bourbon list modal, and stat improvements

- Add bottle_size field to User model and UserResponse/UserUpdate schemas
- Settings modal includes bottle size input (shots capacity)
- Community bottles and My Bottle page show fill bar based on bottle size
- Community bottle cards are clickable — opens searchable bourbon list modal
- Add total_shots_added stat to replace duplicate net volume on dashboard
- Reorder dashboard stats: Bourbons Added, Total Poured In, Shots Remaining, Est. Proof
- Theme-matched custom scrollbar (amber on dark)

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

backend/app/schemas/user.py

@@ -12,6 +12,7 @@ class UserCreate(BaseModel):
 class UserUpdate(BaseModel):
     display_name: Optional[str] = None
     timezone: Optional[str] = None
+    bottle_size: Optional[float] = None
 
 
 class PasswordChange(BaseModel):
@@ -24,6 +25,7 @@ class UserResponse(BaseModel):
     email: str
     display_name: Optional[str]
     timezone: str
+    bottle_size: Optional[float]
     is_admin: bool
     created_at: datetime