fix: render correct meta key in keyboard hints on Windows/Linux (#168)

Hardcoded ⌘ glyphs in compose hints were misleading on non-mac
platforms. Add a tiny platform helper sourced from the existing
window.electronWindow.platform bridge and substitute "Ctrl" off mac.
Closes #167.

https://claude.ai/code/session_01YZY4wUqACT7mgibHr2Yncx

Co-authored-by: Claude <[email protected]>

js/src/lib/platform.ts

@@ -0,0 +1,5 @@
+export const isMac = window.electronWindow?.platform === "darwin";
+
+// Symbol to show in keyboard hints for the primary modifier
+// (Cmd on macOS, Ctrl on Windows/Linux).
+export const metaKey = isMac ? "⌘" : "Ctrl";