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/prompt.py

@@ -63,6 +63,14 @@ def _prepare_context(prefix: str, suffix: str) -> Tuple[str, str]:
 
 
 FENCE_LINE_RE = re.compile(r"^[ \t]*```.*$")
+FENCE_INFO_RE = re.compile(r"^[ \t]*```[ \t]*(.*)$")
+MERMAID_CONTEXT_RE = re.compile(
+    r"```[ \t]*mermaid\b|"
+    r"\b(flowchart|sequencediagram|classdiagram|statediagram(?:-v2)?|"
+    r"erdiagram|journey|gantt|pie|mindmap|timeline|gitgraph|quadrantchart|xychart-beta)\b|"
+    r"\bgraph[ \t]+(TD|TB|BT|RL|LR)\b",
+    re.IGNORECASE,
+)
 
 
 def _cursor_in_fenced_code_block(prefix: str) -> bool:
@@ -71,12 +79,48 @@ def _cursor_in_fenced_code_block(prefix: str) -> bool:
     The state is computed by toggling on each markdown fence line that matches:
     ^[ \t]*```.*$
     """
+    return _active_fence_language(prefix) != "none"
+
+
+def _active_fence_language(prefix: str) -> str:
+    """
+    Return active fence language at cursor based on prefix.
+    - "none": cursor is outside fenced code block
+    - "unknown": cursor is inside a fence without language tag
+    - "<language>": cursor is inside a fenced block with language tag
+    """
     normalized = _normalize_newlines(prefix)
     in_fence = False
+    active_language = "none"
     for line in normalized.split("\n"):
         if FENCE_LINE_RE.match(line):
-            in_fence = not in_fence
-    return in_fence
+            if in_fence:
+                in_fence = False
+                active_language = "none"
+            else:
+                info_match = FENCE_INFO_RE.match(line)
+                info = info_match.group(1).strip() if info_match else ""
+                if not info:
+                    active_language = "unknown"
+                else:
+                    first_token = info.split()[0]
+                    lang_chars = []
+                    for ch in first_token.strip():
+                        if ch.isalnum() or ch in "-_+.":
+                            lang_chars.append(ch)
+                    active_language = "".join(lang_chars)[:32].lower() or "unknown"
+                in_fence = True
+    return active_language if in_fence else "none"
+
+
+def _is_mermaid_context(prefix: str, suffix: str, cursor_fence_language: str) -> bool:
+    if cursor_fence_language == "mermaid":
+        return True
+
+    prefix_tail = (prefix or "")[-1200:]
+    suffix_head = (suffix or "")[:400]
+    combined = f"{prefix_tail}\n{suffix_head}"
+    return MERMAID_CONTEXT_RE.search(combined) is not None
 
 
 def prepare_prompt_context(prefix: str, suffix: str) -> Tuple[str, str]:
@@ -113,22 +157,36 @@ Hard constraints you must follow:
   - Do not output code snippets as inline backticks.
   - Choose the language tag from context (no default fallback tag instruction).
 
-4) Boundary newline repair:
+4) Mermaid-specific completion rules:
+- Read CURSOR_FENCE_LANGUAGE and MERMAID_CONTEXT from the user prompt.
+- If CURSOR_FENCE_LANGUAGE=mermaid:
+  - Output Mermaid statements only.
+  - Never output triple backticks.
+  - Never output prose explanations.
+- If CURSOR_IN_FENCED_CODE_BLOCK=false and MERMAID_CONTEXT=true:
+  - Output a complete Mermaid fenced block:
+    ```mermaid
+    ...
+    ```
+  - Keep Mermaid syntax valid and concise.
+- Never mix Mermaid code and explanatory narration in one output.
+
+5) Boundary newline repair:
 - Read PREFIX_ENDS_WITH_NEWLINE and SUFFIX_STARTS_WITH_NEWLINE from the user prompt.
 - Carefully reason about whether OUTPUT should start or end with a newline.
 - If PREFIX lacks a required boundary newline, add it at OUTPUT start.
 - If SUFFIX lacks a required boundary newline, add it at OUTPUT end.
 - Ensure PREFIX + OUTPUT + SUFFIX is structurally natural.
 
-5) Context stitching:
+6) Context stitching:
 - Do not repeat text that already appears at the start of SUFFIX.
 - Preserve nearby language, tone, punctuation, indentation, and markdown structure.
 - Continue existing structures naturally (lists, tables, block quotes, headings).
 
-6) OCR safety:
+7) OCR safety:
 - PREFIX may include hidden OCR metadata tags like <OCR:...>.
 - Never output any OCR tag.
-- Never output strings containing <OCR: or > as OCR artifacts."""
+- Never output OCR tag fragments such as <OCR:...>."""
     return system_prompt.strip()
 
 
@@ -227,6 +285,29 @@ The area is $A = \\pi r^2$.
 
 ```javascript
 const area = (r) => Math.PI * r * r;
+```
+
+[EX13] Cursor inside mermaid fence: no backticks, mermaid lines only
+CURSOR_IN_FENCED_CODE_BLOCK=true
+CURSOR_FENCE_LANGUAGE=mermaid
+<PREFIX>```mermaid
+flowchart TD
+A[Start] --> </PREFIX>
+<SUFFIX>
+```</SUFFIX>
+Expected OUTPUT:
+B{Valid?}
+B -->|Yes| C[Done]
+
+[EX14] Mermaid context outside fence: return full mermaid block
+CURSOR_IN_FENCED_CODE_BLOCK=false
+MERMAID_CONTEXT=true
+<PREFIX>Please provide a simple release pipeline diagram.</PREFIX>
+<SUFFIX></SUFFIX>
+Expected OUTPUT:
+```mermaid
+flowchart LR
+Build --> Test --> Deploy
 ```"""
 
 
@@ -243,7 +324,11 @@ def build_completion_prompts(
     recent_prefix = _normalize_newlines(recent_prefix)
     recent_suffix = _normalize_newlines(recent_suffix)
 
-    cursor_in_fenced_code_block = _cursor_in_fenced_code_block(recent_prefix)
+    cursor_fence_language = _active_fence_language(recent_prefix)
+    cursor_in_fenced_code_block = cursor_fence_language != "none"
+    mermaid_context = _is_mermaid_context(
+        recent_prefix, recent_suffix, cursor_fence_language
+    )
     prefix_ends_with_newline = recent_prefix.endswith("\n")
     suffix_starts_with_newline = recent_suffix.startswith("\n")
 
@@ -268,6 +353,8 @@ Editor language id: {safe_language_id}
 
 Completion state flags:
 - CURSOR_IN_FENCED_CODE_BLOCK: {"true" if cursor_in_fenced_code_block else "false"}
+- CURSOR_FENCE_LANGUAGE: {cursor_fence_language}
+- MERMAID_CONTEXT: {"true" if mermaid_context else "false"}
 - PREFIX_ENDS_WITH_NEWLINE: {"true" if prefix_ends_with_newline else "false"}
 - SUFFIX_STARTS_WITH_NEWLINE: {"true" if suffix_starts_with_newline else "false"}