mission canvas: add Refine button + markdown-rendered description

Adds a Refine button to the left of Refresh + Launch on the mission
detail toolbar (draft-only). Clicking it POSTs to a new endpoint that
calls Gemini 2.5 Flash to rewrite the user's freeform description into
a coherent, sectioned Markdown brief (Objective / Context / Scope /
Constraints / Acceptance Criteria / Open Questions) ready for the
research + coding agents to ingest cleanly.

Backend:
  - crates/cm-api/src/mission_refiner.rs — Gemini call with a
    system prompt that preserves user-provided facts, avoids
    invention, and emits raw markdown (not JSON).
  - POST /api/missions/{id}/refine — draft-only, 400 on empty
    description or non-draft state.
  - cm-db::repo::missions::set_description helper.

Frontend:
  - MarkdownBlock — tiny zero-dep renderer for h1/h2/h3, bullet +
    numbered lists, **bold**, `code`, paragraphs. Deliberately
    small; the refiner emits a bounded subset.
  - MissionCanvas — Refine button (Sparkles icon, secondary style)
    to the left of Refresh; description now renders through
    MarkdownBlock instead of a single <p>. Disabled while
    description is empty or a refine is in flight.
  - lib/api/missions — refineMission client.

crates/cm-db/src/repo/missions.rs

@@ -233,6 +233,25 @@ pub async fn list_by_workspace(
         .collect())
 }
 
+pub async fn set_description(
+    pool: &PgPool,
+    id: Uuid,
+    workspace_id: Uuid,
+    description: &str,
+) -> Result<(), DbError> {
+    sqlx::query(
+        "UPDATE missions
+            SET description = $3, updated_at = now()
+          WHERE id = $1 AND workspace_id = $2",
+    )
+    .bind(id)
+    .bind(workspace_id)
+    .bind(description)
+    .execute(pool)
+    .await?;
+    Ok(())
+}
+
 pub async fn set_status(
     pool: &PgPool,
     id: Uuid,