fix: scope backup_diff to server sections and mask secrets

The diff preview in RestoreDialog was serializing the full config dict,
exposing secret env values and token args in cleartext on a pasteable surface.

- Add _redact_server_data / _redact_servers_block helpers that apply
  redact_args to args and mask env values for is_secret_key() keys
- Rewrite backup_diff to compare only {mcpServers, _disabledMcpServers}
  sections (sanitized), not the whole file — also avoids double-serializing
  multi-MB ~/.claude.json for a servers-only diff
- Add clarifying comment in _restore_from_backup about why full_config
  is the right base after confirm-discard
- Add test: backup_diff with secret args/env → MASK in output, raw values absent
- Add test: restore_backup restores _disabledMcpServers correctly

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

bcc_core.py

@@ -273,18 +273,37 @@ def backup_label(backup_path: Path | str) -> str:
     return f"{ts[:4]}-{ts[4:6]}-{ts[6:8]}  {ts[9:11]}:{ts[11:13]}:{ts[13:]}"
 
 
+def _redact_server_data(data: dict) -> dict:
+    """Return a copy of a server definition with secrets masked for display."""
+    out = dict(data)
+    if "args" in out:
+        out["args"] = redact_args(list(out["args"] or []))
+    if "env" in out:
+        out["env"] = {k: (MASK if is_secret_key(k) else v) for k, v in (out["env"] or {}).items()}
+    return out
+
+
+def _redact_servers_block(block: dict | None) -> dict:
+    """Return a sanitized copy of a mcpServers / _disabledMcpServers block."""
+    if not block:
+        return {}
+    return {name: _redact_server_data(data) for name, data in block.items()}
+
+
 def backup_diff(
     config_path: Path | str,
     backup_path: Path | str,
     current_cfg: dict | None = None,
 ) -> str:
     """
-    Unified diff showing what the current config would look like after restoring
-    the backup (servers only — non-server keys are preserved).
+    Unified diff showing what the server sections would look like after restoring
+    the backup. Only mcpServers and _disabledMcpServers are compared; non-server
+    keys are excluded entirely (they are preserved by restore, not changed).
+    Secret values in args and env are masked so the diff is safe to share.
 
     fromfile = current state, tofile = what will be written after restore.
-    Pass current_cfg to use an already-loaded (possibly repaired) in-memory config
-    as the baseline instead of re-reading from disk.
+    Pass current_cfg to diff against an already-loaded in-memory config instead
+    of re-reading from disk.
     """
     p = Path(config_path)
     bp = Path(backup_path)
@@ -299,12 +318,18 @@ def backup_diff(
     after_cfg = dict(current_cfg)
     apply_servers(after_cfg, backup_servers)
 
-    before_lines = (json.dumps(current_cfg, indent=2, ensure_ascii=False) + "\n").splitlines(
-        keepends=True
-    )
-    after_lines = (json.dumps(after_cfg, indent=2, ensure_ascii=False) + "\n").splitlines(
-        keepends=True
-    )
+    def _server_sections(cfg: dict) -> dict:
+        out: dict = {"mcpServers": _redact_servers_block(cfg.get("mcpServers"))}
+        if DISABLED_KEY in cfg:
+            out[DISABLED_KEY] = _redact_servers_block(cfg.get(DISABLED_KEY))
+        return out
+
+    before_lines = (
+        json.dumps(_server_sections(current_cfg), indent=2, ensure_ascii=False) + "\n"
+    ).splitlines(keepends=True)
+    after_lines = (
+        json.dumps(_server_sections(after_cfg), indent=2, ensure_ascii=False) + "\n"
+    ).splitlines(keepends=True)
 
     result = "".join(
         difflib.unified_diff(