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_prompt.py

@@ -21,7 +21,13 @@ def test_prompt_builds_system_and_user():
     assert "$...$" in system_prompt
     assert "$$...$$" in system_prompt
     assert "```{language}" in system_prompt
+    assert "Mermaid-specific completion rules" in system_prompt
+    assert "CURSOR_FENCE_LANGUAGE" in system_prompt
+    assert "MERMAID_CONTEXT" in system_prompt
+    assert "Output Mermaid statements only." in system_prompt
     assert "CURSOR_IN_FENCED_CODE_BLOCK" in user_prompt
+    assert "CURSOR_FENCE_LANGUAGE" in user_prompt
+    assert "MERMAID_CONTEXT" in user_prompt
     assert "PREFIX_ENDS_WITH_NEWLINE" in user_prompt
     assert "SUFFIX_STARTS_WITH_NEWLINE" in user_prompt
 
@@ -33,12 +39,22 @@ def test_cursor_in_fence_detection():
     assert prompt._cursor_in_fenced_code_block("text ```not-a-fence``` tail") is False
 
 
+def test_active_fence_language_detection():
+    assert prompt._active_fence_language("") == "none"
+    assert prompt._active_fence_language("```mermaid\nflowchart TD\nA-->B\n") == "mermaid"
+    assert prompt._active_fence_language("```python\nprint('x')\n") == "python"
+    assert prompt._active_fence_language("```\nline\n") == "unknown"
+    assert prompt._active_fence_language("```mermaid\nA-->B\n```\n") == "none"
+
+
 def test_newline_flags():
     _, user_prompt_a = prompt.build_completion_prompts(
         prefix="Hello",
         suffix="World",
     )
     assert "CURSOR_IN_FENCED_CODE_BLOCK: false" in user_prompt_a
+    assert "CURSOR_FENCE_LANGUAGE: none" in user_prompt_a
+    assert "MERMAID_CONTEXT: false" in user_prompt_a
     assert "PREFIX_ENDS_WITH_NEWLINE: false" in user_prompt_a
     assert "SUFFIX_STARTS_WITH_NEWLINE: false" in user_prompt_a
 
@@ -46,11 +62,30 @@ def test_newline_flags():
         prefix="Hello\n",
         suffix="\nWorld",
     )
+    assert "CURSOR_FENCE_LANGUAGE: none" in user_prompt_b
     assert "PREFIX_ENDS_WITH_NEWLINE: true" in user_prompt_b
     assert "SUFFIX_STARTS_WITH_NEWLINE: true" in user_prompt_b
 
 
+def test_mermaid_context_flags():
+    _, prompt_in_mermaid = prompt.build_completion_prompts(
+        prefix="```mermaid\nflowchart TD\nA --> ",
+        suffix="\n```",
+    )
+    assert "CURSOR_IN_FENCED_CODE_BLOCK: true" in prompt_in_mermaid
+    assert "CURSOR_FENCE_LANGUAGE: mermaid" in prompt_in_mermaid
+    assert "MERMAID_CONTEXT: true" in prompt_in_mermaid
+
+    _, prompt_mermaid_keyword = prompt.build_completion_prompts(
+        prefix="Please draw a mermaid flowchart for deploy pipeline.",
+        suffix="",
+    )
+    assert "CURSOR_IN_FENCED_CODE_BLOCK: false" in prompt_mermaid_keyword
+    assert "CURSOR_FENCE_LANGUAGE: none" in prompt_mermaid_keyword
+    assert "MERMAID_CONTEXT: true" in prompt_mermaid_keyword
+
+
 def test_examples_coverage():
     _, user_prompt = prompt.build_completion_prompts(prefix="", suffix="")
-    for ex in range(1, 13):
+    for ex in range(1, 15):
         assert f"[EX{ex:02d}]" in user_prompt