refine polish: before/after diff view + accept/cancel/restore

Refine no longer clobbers the mission description on click. Flow:
  1. Click Refine → server generates the rewrite, returns
     { original, refined } WITHOUT persisting
  2. RefineDiffModal shows a side-by-side pane (raw before,
     Markdown-rendered after)
  3. User picks:
     - Accept → PATCH /api/missions/{id}/description commits refined
     - Cancel → discards the proposal, description unchanged
     - Restore original → forces a write of `original` (undo path
       for accidentally-accepted refines, since Accept+Cancel is
       still a two-step confirmation)

Backend:
  - mission_refiner::refine returns a RefineResult { original, refined }
    struct instead of persisting + returning the text
  - routes::missions::refine now returns { original, refined }
  - routes::missions::set_description added on PATCH
    /api/missions/{id}/description (draft-only)

Frontend:
  - lib/api/missions — refineMission return type is now RefineResult;
    added setMissionDescription
  - MissionCanvas — RefineDiffModal + DiffPane subcomponents;
    accept / cancel / restore handlers wired to state

Closes task #20.

crates/cm-api/src/routes/missions.rs

@@ -221,15 +221,22 @@ pub async fn trigger_security_scan(
     Ok(Json(SecurityScanResponse { findings, tasks }))
 }
 
-/// POST /api/missions/{id}/refine — rewrite the description into a
-/// coherent, sectioned Markdown brief ready for downstream agent
-/// ingestion. Draft-only.
+#[derive(Debug, Serialize)]
+pub struct RefineResponse {
+    pub original: String,
+    pub refined: String,
+}
+
+/// POST /api/missions/{id}/refine — generate a coherent, sectioned
+/// Markdown rewrite of the current description WITHOUT persisting.
+/// Frontend renders a before/after diff; user hits Accept (PATCH
+/// /description) or Cancel. Draft-only.
 pub async fn refine(
     State(state): State<AppState>,
     Authed(user): Authed,
     Path(id): Path<Uuid>,
-) -> Result<Json<Mission>, ApiError> {
-    crate::mission_refiner::refine(&state.pool, user.workspace_id, id)
+) -> Result<Json<RefineResponse>, ApiError> {
+    let result = crate::mission_refiner::refine(&state.pool, user.workspace_id, id)
         .await
         .map_err(|e| {
             eprintln!("mission {id}: refine failed: {e}");
@@ -241,6 +248,39 @@ pub async fn refine(
                 ApiError::Internal
             }
         })?;
+    Ok(Json(RefineResponse {
+        original: result.original,
+        refined: result.refined,
+    }))
+}
+
+#[derive(Debug, Deserialize)]
+pub struct SetDescriptionRequest {
+    pub description: String,
+}
+
+/// PATCH /api/missions/{id}/description — commit a new description.
+/// Draft-only. Used by the Refine Accept flow (and any future
+/// direct-edit surface).
+pub async fn set_description(
+    State(state): State<AppState>,
+    Authed(user): Authed,
+    Path(id): Path<Uuid>,
+    Json(body): Json<SetDescriptionRequest>,
+) -> Result<Json<Mission>, ApiError> {
+    let mission = cm_db::repo::missions::get(&state.pool, id, user.workspace_id.as_uuid())
+        .await?
+        .ok_or(ApiError::NotFound)?;
+    if mission.status != "draft" {
+        return Err(ApiError::BadRequest);
+    }
+    cm_db::repo::missions::set_description(
+        &state.pool,
+        id,
+        user.workspace_id.as_uuid(),
+        &body.description,
+    )
+    .await?;
     let mission = cm_db::repo::missions::get(&state.pool, id, user.workspace_id.as_uuid())
         .await?
         .ok_or(ApiError::NotFound)?;