feat(files): wire askConflict into the routed new-file 409 path

The is_new branch of routedSaveClicked in editor.js used to alert on
409 and force the user to manually pick a different filename. Restore
the overwrite / keep-both / cancel prompt the legacy openEditorTextNew
flow had (via askConflict, lost when Step 9 deleted legacySaveClicked).

Flow on 409:
  * "overwrite" → re-POST /save with the same path. /save overwrites
    in place when the destination is a regular file.
  * "keep-both" → compose a suffixed path via fo.withCollisionSuffix
    (now multi-extension-aware after F3) and POST that.
  * "cancel" → leave the routed modal open with the user's typed
    content intact so they can edit the filename and retry.

Defensively gates the askConflict + withCollisionSuffix calls on
typeof === "function" so older bookmarks (or a dev environment with
one of the modules missing) fall back to an alert rather than a
TypeError. The 409 alert branch is preserved for that path.

Note on when /save actually 409s: regular-file collisions overwrite
silently (200). 409 fires only when the new path collides with a
directory (or a symlink, or a non-file fs entry) — same contract as
the legacy flow had.

Verified live on /overlays/2 in Chromium with a real round-trip:
  1. Click "+ new folder" → create tmp_409_probe
  2. Click "+ new file" → type "tmp_409_probe" → click Create
  3. /save returns 409 (destination is not a file) → askConflict
     opens with the colliding path displayed
  4. Routed modal stays open behind the conflict dialog (typed
     content preserved)
  5. Cancel on conflict → conflict closes, routed modal still open
  6. Cleanup: delete the tmp_409_probe folder via the action API
  * No console errors throughout
  * Demo overlay state unchanged after cleanup

pytest stays at 577 passed, 1 skipped, 3 deselected (no Python
changes in F4).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

l4d2web/l4d2web/static/js/files-overlay/editor.js

@@ -181,7 +181,10 @@
       : ta.value;
 
     // is_new mode: relPath is empty; compose path from data-at-folder
-    // + filename. The /save endpoint creates the file when missing.
+    // + filename. The /save endpoint creates the file when missing,
+    // 409s when the destination already exists. On 409 we offer the
+    // same overwrite / keep-both / cancel prompt that the legacy
+    // create-new flow used (via askConflict in dialogs.js).
     if (!relPath) {
       if (!editedFilename) return;
       const atFolder = ta.dataset.atFolder || "";
@@ -192,11 +195,35 @@
       if (r.ok) {
         if (typeof window.modals?.closeRouted === "function") window.modals.closeRouted();
         scheduleRefresh(parentOf(fullPath));
-      } else if (r.status === 409) {
+        return;
-        alert(r.rawText || `A file at ${fullPath} already exists. Pick a different name.`);
-      } else {
-        alert((r.body && r.body.error) || r.rawText || `Create failed (HTTP ${r.status}).`);
       }
+      if (r.status === 409 && typeof fo.askConflict === "function") {
+        const action = await fo.askConflict(fullPath);
+        if (action === "overwrite") {
+          // /save overwrites in place when the destination is a file —
+          // a plain re-POST does the right thing.
+          const r2 = await postJson(`${baseUrl}/files/save`, { path: fullPath, content });
+          if (r2.ok) {
+            if (typeof window.modals?.closeRouted === "function") window.modals.closeRouted();
+            scheduleRefresh(parentOf(fullPath));
+          } else {
+            alert((r2.body && r2.body.error) || `Save failed (HTTP ${r2.status}).`);
+          }
+        } else if (action === "keep-both" && typeof fo.withCollisionSuffix === "function") {
+          const altered = fo.withCollisionSuffix(fullPath);
+          const r2 = await postJson(`${baseUrl}/files/save`, { path: altered, content });
+          if (r2.ok) {
+            if (typeof window.modals?.closeRouted === "function") window.modals.closeRouted();
+            scheduleRefresh(parentOf(altered));
+          } else {
+            alert((r2.body && r2.body.error) || `Save failed (HTTP ${r2.status}).`);
+          }
+        }
+        // "cancel" → leave the modal open so the user can edit the
+        // filename and try again without losing typed content.
+        return;
+      }
+      alert((r.body && r.body.error) || r.rawText || `Create failed (HTTP ${r.status}).`);
       return;
     }