Change default task db path to ~/.local/share/jr/jr.db

Modify getTasksDbPath in Omni/Task/Core.hs to use XDG data directory (~/.local/share/jr/jr.db) as the default instead of _/tmp/tasks.db. Use System.Directory.getXdgDirectory XdgData. Create directory if it doesn't exist. Keep TASK_DB_PATH env override and TASK_TEST_MODE for tests.