Add content negotiation for /users and /objects endpoints

- /users/{username}: Redirects to /ui/user/{username} for browsers (Accept: text/html)
- /objects/{hash}: Redirects to /ui/asset/{name} for browsers
- APIs still get JSON (application/activity+json) as before

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

server.py

@@ -1383,10 +1383,19 @@ async def webfinger(resource: str):
 
 @app.get("/users/{username}")
 async def get_actor(username: str, request: Request):
-    """Get actor profile for any registered user."""
+    """Get actor profile for any registered user. Content negotiation: HTML for browsers, JSON for APIs."""
     if not user_exists(username):
         raise HTTPException(404, f"Unknown user: {username}")
 
+    # Check Accept header for content negotiation
+    accept = request.headers.get("accept", "")
+    wants_html = "text/html" in accept and "application/json" not in accept and "application/activity+json" not in accept
+
+    if wants_html:
+        # Redirect to UI page for browsers
+        from fastapi.responses import RedirectResponse
+        return RedirectResponse(url=f"/ui/user/{username}", status_code=303)
+
     actor = load_actor(username)
 
     # Add ActivityPub context
@@ -1774,13 +1783,22 @@ async def get_activities():
 
 
 @app.get("/objects/{content_hash}")
-async def get_object(content_hash: str):
-    """Get object by content hash."""
+async def get_object(content_hash: str, request: Request):
+    """Get object by content hash. Content negotiation: HTML for browsers, JSON for APIs."""
     registry = load_registry()
 
     # Find asset by hash
     for name, asset in registry.get("assets", {}).items():
         if asset.get("content_hash") == content_hash:
+            # Check Accept header for content negotiation
+            accept = request.headers.get("accept", "")
+            wants_html = "text/html" in accept and "application/json" not in accept and "application/activity+json" not in accept
+
+            if wants_html:
+                # Redirect to UI page for browsers
+                from fastapi.responses import RedirectResponse
+                return RedirectResponse(url=f"/ui/asset/{name}", status_code=303)
+
             owner = asset.get("owner", "unknown")
             return JSONResponse(
                 content={