feat(studio): "Explain this" + System AI in Settings (v0.11.0)

Two new operator affordances powered by `HubAdmin.AskAi`:

  * **Audit drill-down → "Explain" button.** Tappable on every
    failed event when System AI is configured; greyed out with
    a "Configure in Settings" tooltip otherwise. Shows an
    inline panel under the event fields with the LLM's plain-
    language explanation, a "FIX" suggestion, the privacy-mode
    badge (`redacted` / `full`), and round-trip latency. Errors
    map to actionable fix hints (`env_missing` → "set $VAR in
    your shell, then fai daemon restart"; `network` → "is your
    provider running?"; etc.) — no raw HTTP errors surfaced.

  * **Settings dialog → System AI panel.** Renders the live
    HubAdmin.SystemAiStatus (provider, endpoint, model,
    api_key_env name, privacy mode). When disabled, shows a
    one-line explainer + pointer to docs/architecture/system-ai.md.
    Read-only for now; editing the config still requires
    ~/.fai/config.yaml (Phase-1+ adds in-place editing).

Both surfaces follow the new zero-learning-curve UX rule:
the user never has to look up what the configurable means or
how to fix an error — every state self-explains in place.

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

pubspec.lock

@@ -79,7 +79,7 @@ packages:
       path: "../fai_dart_sdk"
       relative: true
     source: path
-    version: "0.3.0"
+    version: "0.4.0"
   fake_async:
     dependency: transitive
     description: