Fix main-document switching end to end; stable project-independent MCP pathHEADmain

Main document: the app never called Overleaf's official settings endpoint,
so "Set as main document" only changed renderer state and agents had no way
to switch at all — compiles kept using the old root. Now:
- Right-click Set as main and the new set_main_file MCP tool both POST
  /project/:id/settings {rootDocId} (web parity, persists project-wide)
- Handle the rootDocUpdated broadcast: update the project snapshot, the
  renderer state, and rewrite CLAUDE.md/AGENTS.md — the guide's stale
  "Main file" line was teaching agents to compile the old root via
  main_file overrides
- Compile root resolution uses the sync bridge's live maps, so docs
  created mid-session resolve correctly (agent guide too)

MCP server location: Codex stores its MCP registration globally, and the
per-project path went stale when the project temp dir was cleaned up
("connection closed" on handshake). The bundled server now installs to
~/.lattex/lattex-mcp.mjs (project-independent, refreshed on connect; the
esbuild bundle since the raw source can't resolve node_modules from there)
and resolves the project at runtime: $LATTEX_PROJECT_DIR, cwd + ancestors,
legacy script location, then single live project in tmpdir. Register once,
works for every project.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

2 files changed, 30 insertions, 5 deletions

diff --git a/src/renderer/src/App.tsx b/src/renderer/src/App.tsx
index f3f9f79..0f48a3c 100644
--- a/src/renderer/src/App.tsx
+++ b/src/renderer/src/App.tsx
@@ -309,6 +309,17 @@ export default function App() {
       setStatusMessage('Overleaf session expired — please sign in again')
     })
 
+    // Main document changed (by us or a collaborator) — track it
+    const unsubRootDoc = window.api.onProjectRootDocUpdated?.((docId) => {
+      const store = useAppStore.getState()
+      store.setMainDocument(docId)
+      if (store.overleafProject) {
+        store.setOverleafProject({ ...store.overleafProject, rootDocId: docId })
+      }
+      const name = store.docPathMap[docId]?.split('/').pop()
+      if (name) setStatusMessage(`Main document: ${name}`)
+    })
+
     // Keep the file tree in sync with Overleaf project-entity socket events.
     const unsubEntityCreated = window.api.onSyncEntityCreated((data) => {
       applyEntityCreated(data)
@@ -427,6 +438,7 @@ export default function App() {
       unsubCursorDisconnected()
       unsubFileStatus?.()
       unsubAuthExpired?.()
+      unsubRootDoc?.()
       remoteCursors.clear()
       stopAutocompleteSync()
     }
diff --git a/src/renderer/src/components/FileTree.tsx b/src/renderer/src/components/FileTree.tsx
index 5486f20..f53ae2f 100644
--- a/src/renderer/src/components/FileTree.tsx
+++ b/src/renderer/src/components/FileTree.tsx
@@ -212,14 +212,27 @@ export default function FileTree() {
 
   // ── Project view actions (Overleaf entities) ──
 
-  const handleSetMainDoc = () => {
+  const handleSetMainDoc = async () => {
     if (!ctxMenu) return
     const node = ctxMenu.node
-    if (node.docId) {
-      useAppStore.getState().setMainDocument(node.docId)
-      useAppStore.getState().setStatusMessage(`Main document set to ${node.name}`)
-    }
     closeMenu()
+    if (!node.docId) return
+    const store = useAppStore.getState()
+    // Optimistic local update; the official settings endpoint persists it
+    // project-wide (and broadcasts rootDocUpdated to collaborators)
+    store.setMainDocument(node.docId)
+    const projectId = store.overleafProjectId
+    if (!projectId) return
+    const r = await window.api.overleafSetRootDoc(projectId, node.docId)
+    if (r.success) {
+      const st = useAppStore.getState()
+      if (st.overleafProject) {
+        st.setOverleafProject({ ...st.overleafProject, rootDocId: node.docId })
+      }
+      st.setStatusMessage(`Main document set to ${node.name}`)
+    } else {
+      useAppStore.getState().setStatusMessage(`Failed to set main document (${r.message || 'error'})`)
+    }
   }
 
   const handleCopyPath = () => {