feat(config): backup v1 file before v2 migration

- Add timestamped backup at `~/.cc-switch/config.v1.backup.<ts>.json`
- Keep provider files untouched; only cc-switch metadata is backed up
- Remove UI notification plan; backup only as requested
- Update CHANGELOG with migration backup notes

CHANGELOG.md

@@ -44,6 +44,11 @@ For users upgrading from v2.x (Electron version):
 - The app will automatically migrate your existing provider configurations
 - Window position and size preferences have been reset to defaults
 
+#### Backup on v1→v2 Migration (cc-switch internal config)
+- When the app detects an old v1 config structure at `~/.cc-switch/config.json`, it now creates a timestamped backup before writing the new v2 structure.
+- Backup location: `~/.cc-switch/config.v1.backup.<timestamp>.json`
+- This only concerns cc-switch's own metadata file; your actual provider files under `~/.claude/` and `~/.codex/` are untouched.
+
 ### 🛠️ Development
 - Added `pnpm typecheck` command for TypeScript validation
 - Added `pnpm format` and `pnpm format:check` for code formatting
@@ -64,4 +69,4 @@ For users upgrading from v2.x (Electron version):
 ### Features
 - Basic provider management
 - Claude Code integration
-- Configuration file handling
+- Configuration file handling