Completes the `login` verb + the UI add/re-login affordances. The
accountActionChannel 'login' action opens ClaudeLoginDialog — a modal hosting a
TerminalPane that runs `CLAUDE_CONFIG_DIR=<dir> claude login`, so the CLI drives
the OAuth browser flow and credentials land in that account's config dir (D-64:
one CLI-initiated browser flow, on explicit action).
TerminalPane gains optional argv/env/cwdOverride (default stays the login
shell), and its pane.spawn carries env when set. The login pane reuses the
TerminalPane *widget* — a code import, not a runtime dependency, since it spawns
through the always-present pane.spawn IPC rather than the terminal extension.
Tests cover the parameterized spawn args and the dialog's host wiring (title,
CLAUDE_CONFIG_DIR, close). Closes T-485.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
