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/models/user.py

@@ -14,6 +14,7 @@ class User(Base):
     password_hash: Mapped[str] = mapped_column(String(255), nullable=False)
     display_name: Mapped[Optional[str]] = mapped_column(String(100))
     timezone: Mapped[str] = mapped_column(String(50), default="UTC")
+    bottle_size: Mapped[Optional[float]] = mapped_column(default=None)
     is_admin: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False)
     is_disabled: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False)
     created_at: Mapped[datetime] = mapped_column(DateTime, server_default=func.now())