Multi-tab plumbing in AccountContext + reusable ConfirmDialog

[saknarajapakshe]

Part of: #23

What to build

Two small, independent pieces that the rest of #23 and #25 rely on.

1. In AccountContext.tsx:
   • Stop addAccount from auto-switching the current tab (drop its internal switchAccount(id) call).
   • On mount, check the URL for ?account=<id> first, before falling back to the stored "active account" — so a freshly opened tab knows which account it belongs to.
2. A small helper, src/nonview/core/openAccountInNewTab.ts:

   export function openAccountInNewTab(accountId: string): void {
     const url = new URL(import.meta.env.BASE_URL, window.location.origin);
     url.searchParams.set("account", accountId);
     window.open(url.toString(), `quicksilver-${accountId}`);
   }

   Use import.meta.env.BASE_URL, not a bare "/" — the app is served under /quicksilver/.
3. A reusable src/view/atoms/ConfirmDialog.tsx — the first confirm dialog in the app, used later for "Sign out of all accounts?" and "Remove this account?".

Done when

• Reloading a tab opened with ?account=<id> still shows that account.
• ConfirmDialog goes full-screen on mobile, centered on desktop (same pattern as ComposeDialog).

Depends on

Nothing beyond #28 — start any time.