Add a dedicated AI tab with provider Select, model Input, base URL Input,
and masked API key Input (write-only, stored in vault). Switching providers
reactively updates the model placeholder, base URL default, and shows/hides
the API key row for cloud vs. local providers. ctrl+s saves config and vault.
Extend GENERAL_FIELDS with Memory, Security, Plugin, and Daemon sections
using a new "section" header type and optional int cast for numeric fields.
_CoreField gains cast: type | None for automatic value coercion on save.
Add 5 new tests covering AI tab rendering, config save, vault key write,
vault key skip-on-empty, and section header rendering.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
