research canvas: wider rail + stage explainer + 409 fix

Three connected fixes from a single session's feedback:

- Structure rail widened from 60→76 px, tabs 42→58 px wide with a
  bit of left padding so labels ("Research", "Visualizations") no
  longer bump against the active-tab indicator strip.
- Research canvas: each stage now shows a small "STAGE · <status>"
  card explaining what state the topic is in and a "Next → …" hint
  describing what the primary button will do. No more guessing
  which of standby/processing/reviewing/publishing means what.
- Request-publish 409 fix:
    - Backend TopicDetail now includes has_pending_publish_request
      (SELECTs pending_for_topic when the topic loads). Frontend
      TopicDetail interface + ResearchCanvas honor the flag: when
      an approval is already pending the "Request publish" button
      is replaced with an amber "Awaiting reviewer approval" pill,
      so double-clicks can't 409 in the first place.
    - runAction() also catches 409 as a signal-of-success (the
      user's intent — "queue for approval" — is satisfied by the
      first attempt), refetches the topic, and lets the new
      awaiting-approval card render instead of surfacing a scary
      error to the user.

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

@@ -138,6 +138,11 @@ pub struct TopicDetail {
     #[serde(flatten)]
     pub topic: cm_db::repo::research_topics::ResearchTopic,
     pub agents: Vec<cm_db::repo::research_topics::AgentSlot>,
+    /// True when a publish approval is already pending for this topic.
+    /// Lets the frontend hide the "Request publish" button and show
+    /// "Awaiting reviewer approval" instead — avoids the 409 the user
+    /// gets from double-clicking.
+    pub has_pending_publish_request: bool,
 }
 
 pub async fn get_topic(
@@ -149,7 +154,15 @@ pub async fn get_topic(
         .await?
         .ok_or(ApiError::NotFound)?;
     let agents = cm_db::repo::research_topics::agents(&state.pool, id).await?;
-    Ok(Json(TopicDetail { topic, agents }))
+    let has_pending_publish_request =
+        cm_db::repo::research_publish_approvals::pending_for_topic(&state.pool, id)
+            .await?
+            .is_some();
+    Ok(Json(TopicDetail {
+        topic,
+        agents,
+        has_pending_publish_request,
+    }))
 }
 
 #[derive(Deserialize)]