Replace all hardcoded `"Zed"` and `"zed"` directory names in
`config_dir()`, `data_dir()`, `state_dir()`, and `temp_dir()` with a
single `APP_NAME` constant in the `paths` crate.

- On macOS/Windows (native paths like Application Support, LocalAppData,
Caches), `APP_NAME` is used directly (`"Zed"`).
- On Linux/FreeBSD (XDG-style paths), `app_name_lowercase()` is used
(`"zed"`).

This ensures forks that change `APP_NAME` automatically get their own
data directories, preventing them from stomping on Zed users' databases
and config.

Release Notes:

- N/A