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>

bcc_core.py

@@ -14,6 +14,7 @@ in its original position.
 from __future__ import annotations
 
 import contextlib
+import difflib
 import functools
 import glob
 import json
@@ -248,6 +249,102 @@ def write_config(path: str | os.PathLike, cfg: dict) -> Path | None:
     return backup
 
 
+# --------------------------------------------------------------------------- #
+# Backup utility functions
+# --------------------------------------------------------------------------- #
+_BACKUP_TS_RE = re.compile(r"(\d{8}-\d{6})")
+
+
+def list_backups(config_path: Path | str) -> list[Path]:
+    """Return backup files for config_path, newest-first. Returns [] if none exist."""
+    p = Path(config_path)
+    bdir = p.parent / BACKUP_DIRNAME
+    if not bdir.is_dir():
+        return []
+    return sorted(bdir.glob(f"{p.stem}.*.json"), reverse=True)
+
+
+def backup_label(backup_path: Path | str) -> str:
+    """Human-readable label derived from the backup filename timestamp."""
+    m = _BACKUP_TS_RE.search(Path(backup_path).name)
+    if not m:
+        return Path(backup_path).name
+    ts = m.group(1)  # e.g. "20260702-004124"
+    return f"{ts[:4]}-{ts[4:6]}-{ts[6:8]}  {ts[9:11]}:{ts[11:13]}:{ts[13:]}"
+
+
+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).
+
+    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.
+    """
+    p = Path(config_path)
+    bp = Path(backup_path)
+
+    if current_cfg is None:
+        try:
+            current_cfg = load_config(p)
+        except Exception:
+            current_cfg = {}
+
+    backup_servers = extract_servers(load_config(bp))
+    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
+    )
+
+    result = "".join(
+        difflib.unified_diff(
+            before_lines,
+            after_lines,
+            fromfile=f"current: {p.name}",
+            tofile=f"restore: {bp.name}",
+        )
+    )
+    return result or "(no differences — backup matches current servers)"
+
+
+def restore_backup(
+    config_path: Path | str,
+    backup_path: Path | str,
+    current_cfg: dict | None = None,
+) -> Path | None:
+    """
+    Restore server entries from backup_path into config_path.
+
+    Only mcpServers and _disabledMcpServers are replaced; all other keys in the
+    current config (conversation history, project state, etc.) are preserved
+    verbatim and in their original order.
+
+    Goes through write_config() so a pre-restore backup of the current file is
+    created automatically. Returns that backup path (or None if no prior file).
+    """
+    p = Path(config_path)
+    if current_cfg is None:
+        try:
+            current_cfg = load_config(p)
+        except Exception:
+            current_cfg = {}
+
+    backup_servers = extract_servers(load_config(Path(backup_path)))
+    cfg = dict(current_cfg)
+    apply_servers(cfg, backup_servers)
+    return write_config(p, cfg)
+
+
 # --------------------------------------------------------------------------- #
 # Paste / import parsing
 # --------------------------------------------------------------------------- #