feat(libei): remember the input capture permission across sessions

[cantona]

What

Remember the InputCapture permission across sessions. When the portal offers InputCapture v2, create the session with CreateSession2 and call Start with persist_mode = ExplicitlyRevoked and the stored restore token. On v1, fall back to CreateSession as before.

• The token lives in $XDG_STATE_HOME/lan-mouse/input-capture-restore-token (falling back to ~/.local/state; only absolute paths are used). It is written with mode 0600 through a per-process temp file and a rename, and the file work runs on spawn_blocking.
• Tokens are single-use, so every Start replaces the stored one. The stored token is dropped when Start fails or answers without a replacement, because ashpd's errors cannot tell whether the portal saw the token.

Why draft

The v2 path is untested. No portal I have access to reports InputCapture v2. On GNOME 50 (xdg-desktop-portal 1.21.1, xdg-desktop-portal-gnome 50.0), the frontend exposes CreateSession2/Start, but the interface version is 1 and the GNOME backend implements no Start. So ashpd returns RequiresVersion(2, 1) and the fallback runs.

Testing

• v1 fallback on GNOME 50: logs InputCapture portal is v1, persistence needs v2: permission cannot be remembered, and capture works as before.
• Checks: cargo fmt --check, clippy -D warnings and test pass locally (workspace minus lan-mouse-gtk, whose system libraries I don't have).

Testing on a portal with InputCapture v2 would be very welcome.

[feschber]

Your load and save token functions are overly complicated.
Please take a look at input-emulation/src/libei.rs.
XDG_CACHE_HOME is also the more appropriate location for this I think.

[cantona]

Thanks, simplified. get_token_file_path / read_token / write_token now follow input-emulation/src/libei.rs, and the token lives in $XDG_CACHE_HOME/lan-mouse/input-capture.token, next to remote-desktop.token. Beyond the RemoteDesktop flow, it only:

• closes the session when Start fails, since ashpd's Session has no Drop;
• skips an empty token file;
• logs the v1 fallback once, because the capture session is recreated on every barrier or device change.

I also added a separate commit that writes both token files with mode 0600. fs::write uses the umask, and my remote-desktop.token was 0664, readable by other local users, while the token lets its holder skip the consent dialog. Happy to drop it or move it to its own PR if you'd rather keep this one to InputCapture.