Give the account its own page in place of the sidebar version

[enaboapps]

Closes #992
Part of #988.

What

• The account has its own page: the version label at the bottom of the sidebar is replaced by an account entry that opens an Account page. The page is titled "Account", with the subtitle "Bring your settings to your other computers", and contains the Switchify account section (sign-in, sync status, Sync now, Sign out, Delete account).
• Highlighting: the entry is highlighted (data-active, aria-current="page") while the Account page is open, like the other sidebar destinations.
• Settings has no Account tab any more: it's back to General, Input, Cursor appearance, Privacy and Updates.
• What the entry shows:
  • Signed in: a person icon, "Account" and the email, shortened with an ellipsis. The accessible name is "Account, signed in as …".
  • Signed out: "Sign in", with the accessible name "Sign in to your account".
  • Keychain locked: a neutral "Account", since a saved sign-in may be hidden.
  • It's a full-size nav-button, and it stays visible in the compact layout under 800 px, where the footer used to be hidden.
• No keychain prompts: the sidebar reads the account with a new read-only get_account_status command, so loading it never re-reads or prompts for the keychain. Opening the Account page still retries the keychain, and its get_account now broadcasts account-changed, so a recovered sign-in reaches the sidebar.
• Live updates: the entry follows account-changed (sign-in, sign-out, delete, a session rejected elsewhere).
• Unsaved profile edits: navigating away still asks first, like any sidebar destination.
• No accounts configured: such builds keep showing the version. It also stays visible in Settings → Updates and on the Home update card.

Validation

• npm run lint, npm test (287 passed) and npm run build. The sidebar and Account page tests cover:
  • the signed-in entry replacing the version
  • signed out showing "Sign in" and following a live change
  • opening the Account page with the entry marked current
  • leaving it for another page
  • the read-only status call
  • the locked keychain
  • a cancelled discard of a profile edit
  • builds without accounts keeping the version
  • Account no longer being a Settings tab
• Rust: one read-only command added. cargo clippy is clean, and cargo test --lib passes (708).
• In the running app on Windows (signed in to the real account; checked by computer use, with screenshots):
  • The sidebar entry shows "Account" and the email.
  • It opens the Account page, highlighted like the other destinations, with "Signed in as …", "Settings are up to date", and Sync now / Sign out / Delete account….
  • Settings shows only General, Input, Cursor appearance, Privacy and Updates.

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