fix(hotkey): split platform files by platform+cgo build matrix

The `!darwin` guard on the modifier helpers was too broad: golang.design/x/
hotkey only exposes its Mod*/Key* constants from the darwin&&cgo, linux&&cgo
and windows backends, while the `!windows && !cgo` complement (e.g. Linux
cross-builds with CGO_ENABLED=0) falls back to a stub with no constants. That
made hk.ModCtrl/ModShift/KeyA... undefined and produced editor/vet warnings.

Realign the build tags with the upstream matrix and confine every hk.* constant
to a tagged file so the shared hotkey.go compiles everywhere. Also fix the
Linux mapping (option -> hk.Mod1; hk.ModAlt does not exist on X11).

internal/infrastructure/hotkey/keys.go

@@ -1,9 +1,15 @@
+//go:build windows || cgo
+
 package hotkey
 
 import hk "golang.design/x/hotkey"
 
 // lookupKey maps a token from the user-supplied hotkey spec to a hk.Key.
 //
+// The Key* constants only exist in the platform backends that ship them
+// (windows, or the cgo-backed darwin/linux implementations); the cgo-less stub
+// in hotkey_unsupported.go provides a fallback lookupKey for the complement.
+//
 // The map is intentionally exhaustive over the alphanumeric set so we never
 // rely on an assumption that hk.KeyA..hk.KeyZ are contiguous values — the
 // upstream package gives no such guarantee.