feat(api): add completion request cancellation and mermaid rendering

Add support for cancelling in-progress LLM completion requests via new /v1/completions/cancel endpoint with task tracking. Implement mermaid diagram rendering in the Milkdown editor with a new mermaidPlugin. Update copilotPlugin to properly abort requests with descriptive reasons. Refactor settings panel to handle system theme changes reactively. Add camera capture support for image uploads.

backend/tests/test_main_cancel.py

@@ -0,0 +1,118 @@
+import asyncio
+import importlib
+import sys
+import threading
+from pathlib import Path
+
+import pytest
+from fastapi.testclient import TestClient
+
+
+BACKEND_DIR = Path(__file__).resolve().parents[1]
+if str(BACKEND_DIR) not in sys.path:
+    sys.path.insert(0, str(BACKEND_DIR))
+
+try:
+    main = importlib.import_module("main")
+except ModuleNotFoundError:
+    pytest.skip("main module dependencies are not available", allow_module_level=True)
+
+
+API_KEY_HEADERS = {"X-API-Key": "your-secret-key-here"}
+
+
+def _completion_payload():
+    return {
+        "prefix": "hello",
+        "suffix": "",
+        "languageId": "markdown",
+        "model_thinking": "low",
+        "privacy_mode": True,
+    }
+
+
+def test_cancel_endpoint_cancels_running_task(monkeypatch):
+    main.ACTIVE_COMPLETIONS.clear()
+    started = threading.Event()
+    cancelled = threading.Event()
+
+    async def fake_call_ollama(*args, **kwargs):
+        started.set()
+        try:
+            while True:
+                await asyncio.sleep(0.05)
+        except asyncio.CancelledError:
+            cancelled.set()
+            raise
+
+    monkeypatch.setattr(main, "call_ollama", fake_call_ollama)
+    monkeypatch.setattr(main, "build_completion_prompts", lambda *a, **k: ("system", "user"))
+    monkeypatch.setattr(main, "prepare_prompt_context", lambda *a, **k: ("prefix", "suffix"))
+
+    with TestClient(main.app) as client:
+        request_id = "req-cancel-1"
+        completion_headers = {**API_KEY_HEADERS, "X-Request-Id": request_id}
+        response_box = {}
+
+        def send_completion():
+            response_box["response"] = client.post(
+                "/v1/completions",
+                headers=completion_headers,
+                json=_completion_payload(),
+            )
+
+        completion_thread = threading.Thread(target=send_completion, daemon=True)
+        completion_thread.start()
+
+        assert started.wait(timeout=2.0)
+
+        cancel_response = client.post(
+            "/v1/completions/cancel",
+            headers=API_KEY_HEADERS,
+            json={"request_id": request_id, "reason": "superseded"},
+        )
+        assert cancel_response.status_code == 200
+        assert cancel_response.json() == {"cancelled": True, "status": "ok"}
+
+        completion_thread.join(timeout=5.0)
+        assert not completion_thread.is_alive()
+        assert cancelled.wait(timeout=2.0)
+
+        completion_response = response_box["response"]
+        assert completion_response.status_code == 200
+        assert '"cancelled": true' in completion_response.text
+
+
+def test_cancel_not_found():
+    main.ACTIVE_COMPLETIONS.clear()
+    with TestClient(main.app) as client:
+        response = client.post(
+            "/v1/completions/cancel",
+            headers=API_KEY_HEADERS,
+            json={"request_id": "missing", "reason": "abort"},
+        )
+    assert response.status_code == 200
+    assert response.json() == {"cancelled": False, "status": "not_found"}
+
+
+def test_completion_normal_flow(monkeypatch):
+    main.ACTIVE_COMPLETIONS.clear()
+
+    async def fake_call_ollama(*args, **kwargs):
+        return {"content": "completion text", "think": ""}
+
+    monkeypatch.setattr(main, "call_ollama", fake_call_ollama)
+    monkeypatch.setattr(main, "build_completion_prompts", lambda *a, **k: ("system", "user"))
+    monkeypatch.setattr(main, "prepare_prompt_context", lambda *a, **k: ("prefix", "suffix"))
+
+    with TestClient(main.app) as client:
+        response = client.post(
+            "/v1/completions",
+            headers=API_KEY_HEADERS,
+            json=_completion_payload(),
+        )
+
+    assert response.status_code == 200
+    assert '"content": "completion text"' in response.text
+    assert '"done": true' in response.text
+    assert main.ACTIVE_COMPLETIONS == {}