fix: use stable ~/.codepilot/ data directory to persist across updates

Previously the database path depended on app.getPath('userData') which
varies with app name changes (CodePilot vs codepilot vs Claude GUI).
Now uses a fixed ~/.codepilot/ directory for both dev and production.

Includes automatic migration: on first launch, if the DB doesn't exist
at the new path, it checks old locations (Application Support variants
and the old dev-mode data/ folder) and copies the database over.

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

electron/main.ts

@@ -116,7 +116,7 @@ function startServer(port: number): ChildProcess {
     ...userShellEnv,
     PORT: String(port),
     HOSTNAME: '127.0.0.1',
-    CLAUDE_GUI_DATA_DIR: app.getPath('userData'),
+    CLAUDE_GUI_DATA_DIR: path.join(home, '.codepilot'),
     ELECTRON_RUN_AS_NODE: '1',
     HOME: home,
     PATH: `${basePath}:${home}/.npm-global/bin:${home}/.local/bin:${home}/.claude/bin:${shellPath}`,