feat(studio): in-place System AI editor (v0.12.0)

Replaces the read-only System AI block in Settings with a real
editor. Operator never has to touch ~/.fai/config.yaml again
to change provider, endpoint, model, API-key env-var, or
privacy mode.

UX choices that align with the zero-learning-curve rule:

  * Provider dropdown lists Ollama / OpenAI / LM Studio / vLLM
    / Custom by friendly name. Each selection auto-fills the
    endpoint and api_key_env defaults — but only when the
    field is still empty or matches a different preset's
    default, so manual overrides are never clobbered.

  * Each provider has a one-line description rendered under
    the dropdown ("Ollama → Local `ollama serve`. Models stay
    on your machine. No API key needed.") and a model-hint
    placeholder ("gemma3:4b · llama3.2:3b · qwen2.5-coder:7b")
    that goes away when a model is typed.

  * Privacy mode is a vertical radio group with each option's
    description in place — no doc-lookup needed.

  * "Test connection" sends an `ok`-ping and renders the
    same error-kind → fix-hint mapping the audit drill-down
    uses.

  * "Save" persists + hot-reloads the hub. No daemon restart
    required. UI status badge flips to "enabled · <privacy>"
    immediately.

`fai_dart_sdk` 0.5.0 carries the underlying `updateSystemAi` /
`testSystemAi` RPCs.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

lib/widgets/widgets.dart

@@ -12,3 +12,4 @@ export 'fai_module_sheet.dart';
 export 'fai_pill.dart';
 export 'fai_settings_dialog.dart';
 export 'fai_status_dot.dart';
+export 'fai_system_ai_editor.dart';