Use platform-native app shortcuts on macOS

[baxen]

Summary

• Require Command (not Control) for app-level shortcuts on macOS while keeping Control on Windows/Linux
• Add shared platform shortcut helper and reuse it across search, settings, sidebar, find, and zoom shortcuts
• Add macOS-only Emacs-style composer shortcuts for Ctrl-B, Ctrl-F, and Ctrl-K

Test Plan

• cd desktop && pnpm exec tsc --noEmit
• cd desktop && pnpm exec biome check src/app/AppShell.tsx src/app/navigation/useBackForwardControls.ts src/app/useWebviewZoomShortcuts.ts src/features/messages/lib/useRichTextEditor.ts src/features/search/useChannelFind.ts src/shared/lib/keyboard-shortcuts.ts src/shared/lib/platform.ts src/shared/ui/sidebar.tsx

[wesbillman]

Nice!