refactor(files): extract _load_file_for_editing helper

Step 11/12 of docs/superpowers/plans/2026-05-17-files-overlay-rewrite.md.

overlay_file_content and overlay_file_edit_page used to repeat the
same prelude: load the files-overlay, resolve sub_path safely, confirm
it's a regular file, attempt a UTF-8 read with the is_editable sniff
+ UnicodeDecodeError-on-tail fallback. The bodies have diverged in
two places (the binary branch differs — content endpoint returns 415,
edit page renders the binary template) but the prelude is identical.

Extract _load_file_for_editing(overlay_id, sub_path, user). Returns
either a Flask Response (400/404/403/500) or a 5-tuple
(overlay, target, content_or_None, is_binary, byte_count). Both
routes call it and translate the tuple into their output shape:

  * overlay_file_content: 415 when is_binary, else jsonify text
  * overlay_file_edit_page: render binary template when is_binary,
    else render text template

The previous _render_binary_editor helper folds back into the route
since it was only called from there and benefits from inlining now
that the prelude is shared.

is_binary in the tuple is True both when is_editable() said no up
front (8-KiB sniff) AND when the tail-of-file read raised
UnicodeDecodeError — collapsing what was a duplicated try/except
branch in each route.

No behavior change. pytest still 580 passed, 1 skipped, 3 deselected.

Step 12 (next) audits /files/content callers and likely deletes the
endpoint since Phase B removed its only caller (legacy
openEditorForFile via files-overlay.js); the shared helper would
then collapse to single-caller status but remains useful for
grouping the prelude into a named concept.

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

l4d2web/l4d2web/routes/files_routes.py

@@ -201,6 +201,48 @@ def server_files_fragment(server_id: int):
     )
 
 
+def _load_file_for_editing(overlay_id: int, sub_path: str, user):
+    """Shared prelude for overlay_file_content + overlay_file_edit_page:
+    load the files-overlay, resolve sub_path safely, confirm it's a
+    regular file, attempt a UTF-8 read.
+
+    Returns either:
+      * a 5-tuple (overlay, target, content, is_binary, byte_count) on
+        success — content is the UTF-8 text in text mode, None in
+        binary mode; byte_count is encoded text length or st_size
+      * a Flask Response on any failure: 400 (path traversal), 404
+        (missing or non-file), 403 (system overlay, non-admin), 500
+        (OS read error)
+
+    is_binary is True both when is_editable(target) said no up front
+    (8-KiB sniff) and when the tail of the file failed UTF-8 decode
+    even though the sniff passed."""
+    result = _load_files_overlay(overlay_id, user)
+    if isinstance(result, Response):
+        return result
+    overlay = result
+
+    try:
+        target = safe_resolve_for_listing(overlay.path, sub_path)
+    except ValueError:
+        return Response("invalid path", status=400)
+
+    if not target.exists() or not target.is_file():
+        return Response(status=404)
+
+    if not is_editable(target):
+        return overlay, target, None, True, target.stat().st_size
+
+    try:
+        content = target.read_text(encoding="utf-8")
+    except OSError:
+        return Response("read failed", status=500)
+    except UnicodeDecodeError:
+        return overlay, target, None, True, target.stat().st_size
+
+    return overlay, target, content, False, len(content.encode("utf-8"))
+
+
 @bp.get("/overlays/<int:overlay_id>/files/content")
 @require_login
 def overlay_file_content(overlay_id: int):
@@ -209,29 +251,13 @@ def overlay_file_content(overlay_id: int):
     assert user is not None
     sub_path = request.args.get("path", "")
 
-    result = _load_files_overlay(overlay_id, user)
+    result = _load_file_for_editing(overlay_id, sub_path, user)
     if isinstance(result, Response):
         return result
-    overlay = result
+    _overlay, _target, content, is_binary, _byte_count = result
 
-    try:
+    if is_binary:
-        target = safe_resolve_for_listing(overlay.path, sub_path)
-    except ValueError:
-        return Response("invalid path", status=400)
-
-    if not target.exists() or not target.is_file():
-        return Response(status=404)
-    if not is_editable(target):
         return Response("not editable", status=415)
-
-    try:
-        content = target.read_text(encoding="utf-8")
-    except OSError:
-        return Response("read failed", status=500)
-    except UnicodeDecodeError:
-        # is_editable sniffed only the first 8 KiB; the tail can still fail.
-        return Response("not editable", status=415)
-
     return jsonify({"path": sub_path, "content": content})
 
 
@@ -240,67 +266,43 @@ def overlay_file_content(overlay_id: int):
 def overlay_file_edit_page(overlay_id: int):
     """Server-rendered editor page. Renders full-page by default or as a
     layoutless modal fragment when the HX-Modal header is set (see the
-    inject_base_layout context processor in app.py)."""
+    inject_base_layout context processor in app.py). Non-editable files
+    render the binary-replace branch instead of returning 415."""
     user = current_user()
     assert user is not None
     sub_path = request.args.get("path", "")
 
-    result = _load_files_overlay(overlay_id, user)
+    result = _load_file_for_editing(overlay_id, sub_path, user)
     if isinstance(result, Response):
         return result
-    overlay = result
+    overlay, target, content, is_binary, byte_count = result
 
-    try:
+    if is_binary:
-        target = safe_resolve_for_listing(overlay.path, sub_path)
-    except ValueError:
-        return Response("invalid path", status=400)
-
-    if not target.exists() or not target.is_file():
-        return Response(status=404)
-
-    if not is_editable(target):
-        return _render_binary_editor(overlay, sub_path, target)
-
-    try:
-        content = target.read_text(encoding="utf-8")
-    except OSError:
-        return Response("read failed", status=500)
-    except UnicodeDecodeError:
-        # is_editable's 8-KiB sniff said yes but the tail was binary.
-        # Fall back to the binary template path.
-        return _render_binary_editor(overlay, sub_path, target)
-
-    return render_template(
-        "overlay_file_editor.html",
-        overlay=overlay,
-        rel_path=sub_path,
-        content=content,
-        byte_count=len(content.encode("utf-8")),
-        is_new=False,
-        is_binary=False,
-        at_folder="",
-        mime_type="",
-    )
-
-
-def _render_binary_editor(overlay, sub_path, target):
-    """Render overlay_file_editor.html in binary-replace mode. Used by
-    overlay_file_edit_page when the target file isn't a UTF-8 text file.
-    The template hides the CM6 textarea + language dropdown and shows a
-    Replace zone + Download link; the save button reads 'Replace'."""
         mime, _enc = mimetypes.guess_type(target.name)
         return render_template(
             "overlay_file_editor.html",
             overlay=overlay,
             rel_path=sub_path,
             content="",
-        byte_count=target.stat().st_size,
+            byte_count=byte_count,
             is_new=False,
             is_binary=True,
             at_folder="",
             mime_type=mime or "application/octet-stream",
         )
 
+    return render_template(
+        "overlay_file_editor.html",
+        overlay=overlay,
+        rel_path=sub_path,
+        content=content,
+        byte_count=byte_count,
+        is_new=False,
+        is_binary=False,
+        at_folder="",
+        mime_type="",
+    )
+
 
 @bp.get("/overlays/<int:overlay_id>/files/new")
 @require_login