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Summary

OAuth sign-in already supports adding any number of accounts per provider, and every successful sign-in is saved as a new credential file. The login page did not show this: every card always offered "Start Sign-In" and only showed a session-scoped "Completed" pill, so users often assumed earlier sign-ins had failed or been lost.

• Add an intro banner explaining that every sign-in adds a new account without replacing existing ones, with a shortcut to the Authentication Files tab.
• Show the saved accounts on each built-in provider card: an account count chip plus up to three account labels (disabled accounts are struck through, the rest collapse into "+N more").
• Clicking the count chip opens Authentication Files already filtered to that provider.
• The primary action reads "Sign In" when the provider has no accounts and "Add Account" when it does; while polling it reads "Waiting for Authorization…".
• Replace the "Completed" pill with inline status messages for waiting, success ("New account added and saved to Authentication Files.") and failure (the error plus "Existing accounts are not affected.").
• Refresh the account summary after a successful sign-in and when the page Refresh button is used. If the core is unavailable the summary is hidden and sign-in still works.
• Add groupOAuthAccountsByProvider in services/authFiles.ts, reusing the existing dedupe and OAuth-file filters, so runtime-only and API key entries are excluded.
• Add zh-CN, en and ja strings; zh-TW is derived from zh-CN as before.

Screenshots

Login page	Waiting for authorization

Dark theme	Count chip opens filtered Authentication Files

Captured with the browser mock (?mock=running).

Validation

• bun run check
• bun test tests/authFiles.test.ts tests/oauthLoginState.test.ts, including a new test for the account grouping helper
• tests/meta-oauth-ui.cjs passes once the stale card count is corrected (see note below)
• tests/devin-ui.cjs passes all OAuth login steps; it then stops at the existing devin-test.json strict-mode assertion in the Authentication Files view, which fails the same way on current dev
• Manually checked the login page in light and dark themes, the waiting state and the filtered Authentication Files shortcut in the browser mock

Note: on current dev, meta-oauth-ui.cjs, devin-ui.cjs and plugin-oauth-providers-ui.cjs already fail their .oauth-card count assertions because of the Vertex card. This PR leaves those assertions alone, since #384 addresses them; it only updates the button and status labels these tests look for.