feat: backup restore UI (#3)

Add list_backups / backup_label / backup_diff / restore_backup to bcc_core,
RestoreDialog to bcc.py, and a "Restore…" button in the profile top bar.

Restore is selective: only mcpServers and _disabledMcpServers are replaced;
all other keys in the config (history, project state) are preserved verbatim.
Goes through write_config() so a pre-restore backup is always created first.

Closes #3

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

tests/test_core.py

@@ -318,3 +318,118 @@ def test_spawn_test_large_stderr_does_not_deadlock():
     assert r["returncode"] == 3
     # stderr is capped, not the full 64 KB
     assert len(r["stderr"]) <= c._STDERR_CAP
+
+
+# --------------------------------------------------------------------------- #
+# 8. Backup restore
+# --------------------------------------------------------------------------- #
+@pytest.fixture
+def config_with_backup(tmp_path):
+    """A config file with two backups already present."""
+    cfgpath = tmp_path / "claude_desktop_config.json"
+    original = {
+        "globalShortcut": "Cmd+Shift+Space",
+        "mcpServers": {"server-v1": {"command": "node", "args": ["v1.js"]}},
+    }
+    cfgpath.write_text(json.dumps(original, indent=2))
+
+    # Create first backup (v1 state) by writing a new version
+    cfg = c.load_config(cfgpath)
+    servers = c.extract_servers(cfg)
+    servers[0] = c.ServerEntry("server-v2", {"command": "node", "args": ["v2.js"]}, True)
+    c.apply_servers(cfg, servers)
+    c.write_config(cfgpath, cfg)
+
+    return cfgpath
+
+
+def test_list_backups_returns_newest_first(config_with_backup):
+    backups = c.list_backups(config_with_backup)
+    assert len(backups) >= 1
+    # Sorted newest-first means descending lexicographic order on timestamps
+    names = [b.name for b in backups]
+    assert names == sorted(names, reverse=True)
+
+
+def test_list_backups_empty_when_no_bdir(tmp_path):
+    cfgpath = tmp_path / "claude_desktop_config.json"
+    cfgpath.write_text("{}")
+    assert c.list_backups(cfgpath) == []
+
+
+def test_backup_label_parses_timestamp(config_with_backup):
+    backup = c.list_backups(config_with_backup)[0]
+    label = c.backup_label(backup)
+    # label should look like "2026-07-02  00:41:24" (date + time, separated by two spaces)
+    assert "-" in label and ":" in label
+    assert len(label) == 20  # "YYYY-MM-DD  HH:MM:SS" = 10 + 2 + 8
+
+
+def test_backup_diff_shows_server_change(config_with_backup):
+    backup = c.list_backups(config_with_backup)[0]
+    diff = c.backup_diff(config_with_backup, backup)
+    # The diff should show the transition back to v1 servers
+    assert "server-v1" in diff or "server-v2" in diff
+
+
+def test_backup_diff_no_diff_when_identical(tmp_path):
+    cfgpath = tmp_path / "cfg.json"
+    content = {"mcpServers": {"s": {"command": "node"}}}
+    cfgpath.write_text(json.dumps(content, indent=2))
+    bdir = cfgpath.parent / c.BACKUP_DIRNAME
+    bdir.mkdir(exist_ok=True)
+    import time as _time
+
+    stamp = _time.strftime("%Y%m%d-%H%M%S")
+    backup_path = bdir / f"cfg.{stamp}.json"
+    backup_path.write_text(json.dumps(content, indent=2))
+    diff = c.backup_diff(cfgpath, backup_path)
+    assert "(no differences" in diff
+
+
+def test_restore_backup_replaces_servers(config_with_backup):
+    # Get the v1 backup (only backup we have)
+    backups = c.list_backups(config_with_backup)
+    assert len(backups) >= 1
+    v1_backup = backups[0]
+
+    # Current file has server-v2; restore should bring back server-v1
+    c.restore_backup(config_with_backup, v1_backup)
+
+    restored = json.loads(config_with_backup.read_text())
+    assert "server-v1" in restored.get("mcpServers", {})
+    assert "server-v2" not in restored.get("mcpServers", {})
+
+
+def test_restore_preserves_non_server_keys(config_with_backup):
+    backups = c.list_backups(config_with_backup)
+    c.restore_backup(config_with_backup, backups[0])
+
+    restored = json.loads(config_with_backup.read_text())
+    # Non-server key must survive
+    assert restored.get("globalShortcut") == "Cmd+Shift+Space"
+
+
+def test_restore_creates_pre_restore_backup(config_with_backup):
+    before_count = len(c.list_backups(config_with_backup))
+    backups = c.list_backups(config_with_backup)
+    c.restore_backup(config_with_backup, backups[0])
+    after_count = len(c.list_backups(config_with_backup))
+    # restore_backup goes through write_config which makes a backup first
+    assert after_count == before_count + 1
+
+
+def test_restore_with_current_cfg_passed(config_with_backup):
+    # When current_cfg is passed in (the GUI's in-memory config), it should be
+    # used instead of re-reading from disk.
+    backups = c.list_backups(config_with_backup)
+    # Simulate the GUI passing a repaired in-memory version
+    in_memory_cfg = {
+        "globalShortcut": "Cmd+Shift+Space",
+        "extraKey": "preserved",
+        "mcpServers": {"server-v2": {"command": "node", "args": ["v2.js"]}},
+    }
+    c.restore_backup(config_with_backup, backups[0], current_cfg=in_memory_cfg)
+    restored = json.loads(config_with_backup.read_text())
+    assert restored.get("extraKey") == "preserved"
+    assert "server-v1" in restored.get("mcpServers", {})