Slack TA Bot Frontend: Plugin UI (connect, configure, logs)

[zaira-bibi]

Implements the frontend plugin UI for the Slack TA Bot, following the existing plugin UI pattern in frontend/plugins/.

Depends on the OAuth connection issue and the Events/RAG issue.

What's included:

• New plugin UI at frontend/plugins/slack/ registered in frontend/lib/plugins/registry.ts
• OAuth connect/disconnect flow: "Connect to Slack" button → redirects to install URL → shows connected workspace name on return
• After OAuth connection, show a document picker (sourced from GET /rag/sources) so the teacher can select which docs this bot can access
• Bot configuration form: bot_name and fallback_message fields, saved via the existing user-plugin config API. Above selection is also saved as allowed_sources in the bot's plugin config
• Response logs table: Paginated view of GET /logs, showing question, answer, RAG match status, timestamp
• Connection status banner: Shows workspace name when connected, prompt to connect when not

Acceptance Criteria

• Plugin appears in the plugins dashboard under "Slack TA Bot"
• "Connect to Slack" button redirects to the Slack install URL returned by GET /oauth/authorize
• After OAuth callback (?connected=true), UI shows connected workspace name and hides the connect button
• A document picker (sourced from GET /rag/sources) enables the teacher to select which docs this bot can access
• "Disconnect" button calls DELETE /oauth/disconnect and resets the UI to disconnected state
• Bot name and fallback message fields are pre-populated from saved config
• Saving config calls the user-plugin config API and shows success/error feedback
• Logs table shows question, answer (or "No match"), RAG match badge, and timestamp
• Logs table paginates correctly (next/prev)
• Logs table shows empty state when no responses yet
• All API errors surface as user-facing error messages (no silent failures)