Load the translation catalog in main, and translate the menu and dialog titles #623

Part of #622. Needs #584. Blocks #625, #627, #628 and #629.

Why

Several modules build their sentences in main and send them to the renderer, and some of those modules run in both processes (pr-checkout.cjs, legacy-site.cjs, merge-in-progress.cjs, wporg-handle.cjs, patch-provenance.cjs, project-type.cjs). Without a catalog in main, wrapping them gives a translated screen showing English sentences from main. Keep the code mappings that exist (prCheckoutRefusal, ticket-trunk-notice.cjs); don't turn the rest into codes.

What to do

1. Move the body of the i18n:locale handler in src/main.js into one memoised localeReply(). The handler returns it, and whenReady awaits it and applies it in main.
2. Apply it with applyLocale() from src/renderer/locale-setup.cjs, passing setLocaleData from @wordpress/i18n and addFilter from @wordpress/hooks. It has no DOM dependency, and the en-XA filters then cover main's strings too.
3. That call must run before Menu.setApplicationMenu and createWindow().
4. Wrap the two labels in src/menu.js ("Open App Log", "Show Logs Folder"). Leave Electron's role menus alone. Translating those means writing every submenu by hand; that's a separate issue if anyone wants it.
5. Wrap the native dialog titles and filter names in main.js: the save-patch dialog (about line 1036), "Choose a patch file" (about 2390) and "Choose the application to open this folder in" (about 3299).

About 10 strings.

Tests

• tests/unit/menu.test.cjs: labels with a pseudo filter.
• tests/unit/ipc-wiring.test.cjs: the handler and main apply the same reply, resolved once.
• Journey: launch with lang: 'en-XA'. Read the Help menu labels with app.evaluate(({ Menu }) => …). Replace dialog.showOpenDialog the way answerFileDialog does and check the title it was given.