feat(public_page): allow unauthenticated browsing with auth-gated interactivity

- Add get_optional_user dependency; dashboard returns guest-safe payload
- AuthGuard only redirects /following and /notifications for guests
- Sidebar hides auth-required nav items and shows Sign In/Register for guests
- Dashboard shows trending bills as "Most Popular" for unauthenticated visitors
- FollowButton opens AuthModal instead of acting when not signed in
- Members page pins followed members at the top for quick unfollowing
- useFollows skips API call and invalidates dashboard on follow/unfollow

Authored-By: Jack Levy

backend/app/core/dependencies.py

@@ -8,6 +8,7 @@ from app.database import get_db
 from app.models.user import User
 
 oauth2_scheme = OAuth2PasswordBearer(tokenUrl="/api/auth/login")
+oauth2_scheme_optional = OAuth2PasswordBearer(tokenUrl="/api/auth/login", auto_error=False)
 
 
 async def get_current_user(
@@ -30,6 +31,19 @@ async def get_current_user(
     return user
 
 
+async def get_optional_user(
+    token: str | None = Depends(oauth2_scheme_optional),
+    db: AsyncSession = Depends(get_db),
+) -> User | None:
+    if not token:
+        return None
+    try:
+        user_id = decode_token(token)
+        return await db.get(User, user_id)
+    except Exception:
+        return None
+
+
 async def get_current_admin(
     current_user: User = Depends(get_current_user),
 ) -> User: