feat: account import/export (encrypted) + dismissible modals

[noidee-dev]

Summary

• Account export/import — export one, several, or all accounts to a portable string, optionally password-encrypted (scrypt + AES-256-GCM). Copy or download the string; import by pasting it or loading a file.
• Import preview — the app detects in the background whether a pasted blob is encrypted, shows the password field only when needed, and previews the accounts (label + provider) before importing. The Import button is gated on a valid preview.
• Name collisions — when an imported account name already exists, a warning + chooser lets you skip, import as copies, or replace the existing account (one choice for all collisions). Colliding rows are flagged in the preview.
• Dismissible modals — a shared Modal wrapper makes all 10 dialogs close via Esc and backdrop click (also resolves the ROADMAP "Escape-to-close" a11y item).

How it works

• Pure node:crypto module (src/main/accounts/accountTransfer.ts): authenticated encryption, fail-closed on wrong/missing password. No new npm dependency.
• IPC: accounts:export, accounts:import (with onDuplicate), accounts:importPreview (dry-run, returns only label+provider — never secrets), plus generic util:saveTextFile/util:openTextFile.
• Renderer: useAccountTransfer hooks; ExportAccountsDialog / ImportAccountsDialog; entry points in the Accounts screen; ui/Modal.
• i18n added across all 6 locales.
• Security: secrets leave the OS keychain only inside the (optionally encrypted) export string; import is all-or-nothing in a single DB transaction; preview never returns secrets.

Design + plans under docs/superpowers/specs|plans/2026-06-16-*.

Test Plan

• npm test — 533 passing (unit/IPC/UI/integration incl. crypto round-trip, duplicate modes, preview, Modal Esc/backdrop)
• npx tsc --noEmit clean · npm run lint 0 errors
• Manual: export with/without password, copy + download; import via paste and via file; encrypted preview after password; name-collision skip/copy/replace; Esc + backdrop close every dialog
• Reviewer: confirm no secret is ever written to a log or returned by the preview

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