feat(mesh): /api/share-to-mesh iframe intent endpoint (Phase 3c)

Marketplace app iframes (Penpot, Gitea, IndeedHub, ...) can POST a file
to /api/share-to-mesh and postMessage the returned CID to the parent
window. The endpoint mirrors /api/blob's body format but adds CORS for
the requesting app origin (any port on host_ip) so proxied apps can
reach it with credentials:'include'. Session cookie is still the primary
auth; the origin check is a sanity guard.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

core/archipelago/src/api/handler/mod.rs

@@ -147,6 +147,36 @@ impl ApiHandler {
         }
     }
 
+    /// Permissive origin check for the share-to-mesh iframe intent: any scheme
+    /// http(s):// followed by the configured host_ip, optionally `:port`. Apps
+    /// proxied under other ports (APP_PORTS) call this from within the same
+    /// node, so they share host_ip but not port. The session cookie still has
+    /// to be valid — this is a sanity check, not the primary auth.
+    fn validate_app_origin(&self, headers: &hyper::HeaderMap) -> Option<String> {
+        let origin = headers.get("origin").and_then(|v| v.to_str().ok())?;
+        // Allow localhost dev server too so the Vite frontend can exercise it.
+        if self.config.dev_mode && origin == "http://localhost:8100" {
+            return Some(origin.to_string());
+        }
+        let host_ip = &self.config.host_ip;
+        let matches = |scheme: &str| -> bool {
+            let prefix = format!("{}{}", scheme, host_ip);
+            if origin == prefix {
+                return true;
+            }
+            let with_port = format!("{}:", prefix);
+            origin.starts_with(&with_port)
+                && origin[with_port.len()..]
+                    .bytes()
+                    .all(|b| b.is_ascii_digit())
+        };
+        if matches("http://") || matches("https://") {
+            Some(origin.to_string())
+        } else {
+            None
+        }
+    }
+
     pub async fn handle_request(
         &self,
         req: Request<hyper::Body>,
@@ -252,6 +282,38 @@ impl ApiHandler {
                 .await
             }
 
+            // Share-to-mesh intent — marketplace app iframes POST a file here
+            // to stage it as a mesh attachment. Same body format as /api/blob
+            // (raw bytes + X-Blob-Mime/X-Blob-Filename headers). The app is
+            // expected to postMessage `{type:'share-to-mesh', cid, ...}` to
+            // its parent window afterwards so the Mesh view can pick it up.
+            // Authenticated by session cookie + a relaxed Origin check (any
+            // port on the archipelago host is allowed, so proxied apps on
+            // their own ports can reach it with credentials:'include').
+            (Method::POST, "/api/share-to-mesh") => {
+                if !self.is_authenticated(&headers).await {
+                    return Ok(Self::unauthorized());
+                }
+                let origin = match self.validate_app_origin(&headers) {
+                    Some(o) => o,
+                    None => {
+                        return Ok(build_response(
+                            StatusCode::FORBIDDEN,
+                            "text/plain",
+                            hyper::Body::from("origin not allowed"),
+                        ))
+                    }
+                };
+                Self::handle_share_to_mesh(
+                    &self.blob_store,
+                    &self.self_pubkey_hex,
+                    &headers,
+                    body_bytes,
+                    &origin,
+                )
+                .await
+            }
+
             // Blob download — peer-facing. No session required; authenticated
             // by HMAC capability token signed when the blob ref was shared.
             (Method::GET, p) if p.starts_with("/blob/") => {