Replace window.confirm with proper dialogs

[rebaserHEAD]

The unsaved-changes confirmations still go through window.confirm: the native-menu command router (file:new, file:newGrid, app:reload in App.tsx) and the in-app MenuBar''s New Map / New Grid handlers. Unlike window.prompt (#14), confirm does work under Electron, but it renders as a bare Chromium dialog that matches neither the native dialogs nor the in-app modal family.

• Desktop: route through a main-process dialog.showMessageBox IPC (same pattern as the unsaved-close guard).
• Browser: use the existing ConfirmModal.
• Same treatment for any future confirm needs; after this, window.confirm/window.prompt should not appear in the codebase outside the browser-fallback in selectTool.

[rebaserHEAD]

While in this area: ConfirmModal.tsx is the one dialog still styled with inline hardcoded hexes instead of the theme tokens (flagged as known drift in the wiki UI Style Guide). Whatever survives of it after the native-dialog split should come out token-styled like PromptModal.