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_windows.go

@@ -0,0 +1,13 @@
+//go:build windows
+
+package hotkey
+
+import hk "golang.design/x/hotkey"
+
+// On Windows we map "cmd" to Ctrl so the same config string ("cmd+shift+a")
+// stays meaningful across platforms; "option" is treated as Alt. The Windows
+// backend does not require cgo, hence the single-tag guard.
+func modCmd() hk.Modifier    { return hk.ModCtrl }
+func modCtrl() hk.Modifier   { return hk.ModCtrl }
+func modOption() hk.Modifier { return hk.ModAlt }
+func modShift() hk.Modifier  { return hk.ModShift }