[devxsubh]

Problem

Clicking Sign in with browser or Create account on the desktop sign-in gate turned the window blank, and the browser never opened.

The gate and boot-error pages (desktop/src/splash.ts) used about:blank#<action> links, which main.ts intercepted in will-navigate. Electron 43 commits about:blank navigations without emitting will-navigate, so the window actually navigated to about:blank#signin. Reproduced over CDP: after the click, location.href was about:blank#signin with an empty document. Retry and Open logs on the boot-error page were broken the same way.

Fix

• splash.ts — buttons carry data-action; a small inline click handler calls window.capscreenDesktop.shellAction(action).
• preload.ts — adds shellAction, sent on a new capscreen:shell-action channel.
• ipc.ts — the handler accepts the action only when the sender frame is a data: URL (the shell's own pages), so the app UI can't trigger it.
• main.ts — handleShellAction routes signin/signup/reopen/cancel to the existing gate flow and logs/retry to the error-page actions. The about:blank interception in will-navigate is removed.

Testing

• pnpm --filter capscreen-desktop run typecheck passes.
• pnpm desktop against the production auth service (https://capscreen-auth.vercel.app): clicking Sign in moves the gate to "Finish signing in in your browser" and opens the portal. Sign-up, email verification and sign-in completed end to end, and the app unlocked.
• Not exercised: the boot-error page's Retry and Open logs buttons. They use the same path but need a forced boot failure to test.

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