fix: hide native window menu bar on non-macOS and add frameless window resize borders

[bnku]

Description

This PR addresses two window UX issues on Linux (and other frameless desktop environments):

1. Unwanted native menu bar: When app.set_menu was called unconditionally, GTK attached a native menu bar (Edit, Settings, Window, Help) to the window. Because Codex Switcher uses a custom dark titlebar with decorations: false, this native menu bar broke the UI design. Now app.set_menu is only invoked on macOS (where a global application menu bar is standard), while non-macOS desktop platforms manage tray/settings via the tray menu and in-app settings modal.
2. Window resizing on frameless windows: Because decorations is set to false, GTK does not provide window borders. When the native menu bar was present, resizing only worked around the menu bar edges. This PR adds core:window:allow-start-resize-dragging permission and a lightweight <WindowResizeBorders /> overlay in the frontend that provides native resize cursor hover cues (n-resize, s-resize, w-resize, e-resize, corners) and delegates resize drag operations via startResizeDragging when not maximized.

Changes

• src-tauri/src/app_menu.rs: Only attach window menu via app.set_menu on macOS; guard macOS-specific menu event handlers.
• src-tauri/capabilities/default.json: Allow core:window:allow-start-resize-dragging.
• src/components/WindowResizeBorders.tsx: Create resize border handles with correct cursors and resize drag triggers.
• src/App.tsx: Mount WindowResizeBorders when running in frameless desktop mode and not maximized.

Testing

• Tested on Linux (GTK / X11).
• Frontend tests & build passed (pnpm build, npm run test:*).
• Rust tests & formatting passed (cargo test, cargo fmt --check).