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/hotkey.go

@@ -104,7 +104,7 @@ func parseSpec(spec string) ([]hk.Modifier, hk.Key, error) {
 		case "option", "alt":
 			mods = append(mods, modOption())
 		case "shift":
-			mods = append(mods, hk.ModShift)
+			mods = append(mods, modShift())
 		default:
 			k, ok := lookupKey(p)
 			if !ok {