feat: offer guided repair when loading a broken config file

If a config file on disk fails strict JSON parsing, BCC now runs it
through the same repair pipeline as pasted snippets and shows a dialog
listing the parse error, each fix it would apply, and a preview of the
resulting file. The user chooses: Repair & load (marks the profile
dirty; the file is only rewritten on Save, after the broken original
is backed up) or Cancel. Unsalvageable files keep the old error path.

repair_config_file() in bcc_core never writes to disk itself.

bcc.py

@@ -632,6 +632,56 @@ class ArgsEdit(QPlainTextEdit):
             n += 1
 
 
+# --------------------------------------------------------------------------- #
+# Config-repair dialog: shown when a config file on disk fails strict JSON
+# parsing but the repair pipeline can salvage it. Lists what's wrong, what
+# would change, and lets the user decide. Nothing touches disk until Save.
+# --------------------------------------------------------------------------- #
+class RepairDialog(QDialog):
+    def __init__(self, parent, path, error: str, notes: list[str], preview: str):
+        super().__init__(parent)
+        self.setWindowTitle("Config file needs repair")
+        self.resize(620, 520)
+        v = QVBoxLayout(self)
+        v.setSpacing(10)
+
+        intro = QLabel(f"<b>{path}</b><br>isn't valid JSON: {error}")
+        intro.setWordWrap(True)
+        v.addWidget(intro)
+
+        fixes_hdr = QLabel("BCC can repair it automatically. Fixes to apply:")
+        v.addWidget(fixes_hdr)
+        fixes = QLabel("\n".join(f"  •  {n}" for n in notes) or "  •  reformat as valid JSON")
+        fixes.setObjectName("muted")
+        fixes.setWordWrap(True)
+        v.addWidget(fixes)
+
+        prev_hdr = QLabel("The file would become:")
+        v.addWidget(prev_hdr)
+        box = QPlainTextEdit()
+        box.setObjectName("diag")
+        box.setReadOnly(True)
+        box.setPlainText(preview)
+        v.addWidget(box, 1)
+
+        note = QLabel(
+            "Nothing is written yet — choosing Repair loads the fixed config into the "
+            "editor. The file is only rewritten when you press Save, and the broken "
+            "original is backed up first."
+        )
+        note.setObjectName("muted")
+        note.setWordWrap(True)
+        v.addWidget(note)
+
+        btns = QDialogButtonBox()
+        ok = btns.addButton("Repair && load", QDialogButtonBox.ButtonRole.AcceptRole)
+        ok.setObjectName("primary")
+        btns.addButton(QDialogButtonBox.StandardButton.Cancel)
+        btns.accepted.connect(self.accept)
+        btns.rejected.connect(self.reject)
+        v.addWidget(btns)
+
+
 # --------------------------------------------------------------------------- #
 # Paste-JSON dialog
 # --------------------------------------------------------------------------- #
@@ -972,15 +1022,27 @@ class MainWindow(QMainWindow):
             return
         if confirm and self.dirty and not self._confirm_discard():
             return
+        repaired = False
         try:
             self.full_config = core.load_config(profile.path)
         except Exception as e:
-            QMessageBox.critical(
-                self,
-                "Could not read config",
-                f"{profile.path}\n\n{e}\n\nFix the file by hand or pick another install.",
-            )
-            return
+            # Strict parse failed — see if the repair pipeline can salvage it,
+            # and let the user decide with full visibility of the changes.
+            try:
+                cfg, notes, preview = core.repair_config_file(profile.path)
+            except Exception:
+                QMessageBox.critical(
+                    self,
+                    "Could not read config",
+                    f"{profile.path}\n\n{e}\n\nThe file couldn't be repaired automatically. "
+                    f"Fix it by hand or pick another install.",
+                )
+                return
+            dlg = RepairDialog(self, profile.path, str(e), notes, preview)
+            if not dlg.exec():
+                return
+            self.full_config = cfg
+            repaired = True
         self.current_profile = profile
         self.servers = core.extract_servers(self.full_config)
         self.dirty = False
@@ -988,6 +1050,12 @@ class MainWindow(QMainWindow):
         self.undo_btn.setEnabled(False)
         self._refresh_tables(select_index=0 if self.servers else -1)
         self._update_status(saved=False)
+        if repaired:
+            self._mark_dirty()
+            self.status.setText(
+                f"{profile.path}  ·  repaired config loaded — press Save to write the "
+                f"fix (the original file will be backed up)"
+            )
 
     # --- table rendering ------------------------------------------------- #
     def _add_row(self, table, master_idx, s):