ui: improve admin console empty states

[X-PG13]

Summary

• add first-run cues next to the admin token controls
• replace blank or low-context admin panels with actionable empty-state cards
• keep the digest, preview, editor, publication, extraction, and source-health panels guided on empty installs
• tighten the narrow viewport layout to avoid horizontal overflow

Verification

• ./.venv-dev/bin/python -m unittest tests.test_web_console -v
• node --check src/ainews/web/app.js
• git diff --check
• make check PYTHON=./.venv-dev/bin/python
• browser smoke test on an empty AINEWS_HOME at desktop width
• Chrome headless CDP smoke test at 390px viewport: no horizontal overflow and 14 empty-state cards rendered

Closes #148