Dashboard: config editor (form + raw JSON)

[matt-bedda]

Overview

Add a Config page to view and edit the familiar config.json from the dashboard.

Two Modes

1. Form mode: Renders config sections as form fields (telegram, claude, sessions, webhooks, etc.)
2. Raw JSON mode: Full JSON editor with syntax highlighting

API

Already exists: GET /api/config (returns sanitized config with masked tokens)

Need to add:

• PUT /api/config - update config (merge with existing, validate, write to disk)
• Config validation before writing (check required fields, valid cron expressions, etc.)

Security

• Mask sensitive fields in form mode (show **** with reveal toggle)
• PUT endpoint should NOT allow changing telegram.botToken or webhooks.token (security risk)
• Show "restart required" warning when changing cron/webhook settings

Dashboard UI

• Toggle between Form/JSON mode
• Form mode: collapsible sections matching config structure
• JSON mode: textarea with monospace font, format/validate buttons
• Save button with confirmation
• Dirty state indicator
• Undo (revert to saved)

Priority: P3