feat(sync): Windows machines work as sync remotes

Remote temp dirs are now created and removed by the remote SubMiner
itself (sync --make-temp / --remove-temp, validated against its own
tmpdir) instead of mktemp/rm, and the flow detects the remote shell
(POSIX, cmd, PowerShell) to pick quoting and SubMiner install-location
candidates - %LOCALAPPDATA% app install, launcher shim, or PATH - with
no POSIX PATH prefix on Windows. Remote temp paths are normalized to
forward slashes for scp and the CLI. Verified end-to-end against a
throwaway sshd with the app binary as the resolved remote command.

launcher/mpv.test.ts

@@ -581,6 +581,8 @@ function makeArgs(overrides: Partial<Args> = {}): Args {
     syncForce: false,
     syncJson: false,
     syncCheck: false,
+    syncMakeTemp: false,
+    syncRemoveTempPath: '',
     syncUi: false,
     logLevel: 'error',
     logRotation: 7,