[misc, android] Build and lint the mobile Go code in CI (#7641)

Nothing in CI compiles the files behind //go:build android or //go:build ios.
The android bridge builds 7 of its 23 files on linux and skips client.go; the
iOS SDK is not built at all. The linter matrix picks a GOOS by picking a runner
OS, so it loads the same file set as the host build and never sees them either.
A type error in client/android/client.go therefore passes every check on its
PR, merges, and is discovered by netbirdio/android-client after sync-tag.yml
fires trigger_android_bump on the release tag.

The new Mobile workflow cross-compiles ./client/android/... for the GOARCH
values gomobile ships and ./client/ios/..., and vets the android bridge. The
new Android and iOS lint jobs run golangci-lint with GOOS/GOARCH in the job
env. No NDK, Xcode or gomobile is needed: these are library packages, so the
compiler type-checks them without a link step, and the dependency graph drags
in the android/ios-tagged files across client/iface, client/internal/dns and
client/internal/routemanager with them.

Linting those files for the first time surfaces one gosec G101 on the SSH
password-required marker. It is a sentinel string the Java side matches on,
not a credential, so it is suppressed at the declaration.

client/android/ssh_client.go

@@ -31,6 +31,8 @@ const (
 // PasswordRequiredMarker tells Java to prompt for a password and retry. It is
 // a string because gomobile flattens errors to their message, so a sentinel
 // value would not survive the binding.
+//
+//nolint:gosec // G101 false positive: a sentinel marker, not a credential
 const PasswordRequiredMarker = "netbird-ssh-password-required"
 
 // HostKeyUnknownMarker tells Java to show the fingerprint and, on confirmation,