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>

tests/test_core.py

@@ -433,3 +433,70 @@ def test_restore_with_current_cfg_passed(config_with_backup):
     restored = json.loads(config_with_backup.read_text())
     assert restored.get("extraKey") == "preserved"
     assert "server-v1" in restored.get("mcpServers", {})
+
+
+def test_backup_diff_masks_secrets(tmp_path):
+    cfgpath = tmp_path / "cfg.json"
+    current = {
+        "mcpServers": {
+            "s": {
+                "command": "node",
+                "args": ["--token", "super-secret"],
+                "env": {"API_KEY": "my-api-key-value"},
+            }
+        }
+    }
+    cfgpath.write_text(json.dumps(current, indent=2))
+    bdir = cfgpath.parent / c.BACKUP_DIRNAME
+    bdir.mkdir()
+    import time as _t
+
+    bp = bdir / f"cfg.{_t.strftime('%Y%m%d-%H%M%S')}.json"
+    bp.write_text(
+        json.dumps(
+            {
+                "mcpServers": {
+                    "s2": {
+                        "command": "python",
+                        "args": ["--api-key", "another-secret"],
+                        "env": {"SECRET_KEY": "backup-secret-value"},
+                    }
+                }
+            },
+            indent=2,
+        )
+    )
+    diff = c.backup_diff(cfgpath, bp)
+    # Raw secret values must not appear in the diff
+    assert "super-secret" not in diff
+    assert "my-api-key-value" not in diff
+    assert "another-secret" not in diff
+    assert "backup-secret-value" not in diff
+    # The mask placeholder must appear (secrets were present in both sides)
+    assert c.MASK in diff
+
+
+def test_restore_backup_restores_disabled_servers(tmp_path):
+    cfgpath = tmp_path / "cfg.json"
+    original = {
+        "mcpServers": {"active": {"command": "node"}},
+        c.DISABLED_KEY: {"parked": {"command": "python"}},
+    }
+    cfgpath.write_text(json.dumps(original, indent=2))
+
+    # Overwrite: add new server, drop the disabled one
+    cfg = c.load_config(cfgpath)
+    servers = c.extract_servers(cfg)
+    servers = [s for s in servers if s.name != "parked"]
+    servers.append(c.ServerEntry("newcomer", {"command": "uvx"}, True))
+    c.apply_servers(cfg, servers)
+    c.write_config(cfgpath, cfg)
+
+    # Restore from the v1 backup
+    backups = c.list_backups(cfgpath)
+    c.restore_backup(cfgpath, backups[0])
+    restored = json.loads(cfgpath.read_text())
+
+    assert "parked" in restored.get(c.DISABLED_KEY, {})
+    assert "active" in restored.get("mcpServers", {})
+    assert "newcomer" not in restored.get("mcpServers", {})