feat(studio): system-ai editor — test-without-save + model picker (v0.13.0)

Three operator-facing improvements driven by Stefan's testing:

  * **Test Connection** runs against the form's *current* values
    and no longer requires saving first. Previously we save-then-
    test which caused a "system AI is disabled" message when the
    form's model field was still empty. Now the test prompt
    travels with the request body so unsaved tweaks can be
    validated.

  * **Model picker** replaces the free-text Model field. A
    Refresh button calls HubAdmin.ListSystemAiModels (provider's
    `/v1/models`); installed models show up as clickable chips
    that fill the field with one tap. The text input stays for
    cases where the operator already knows the id.

  * **Pull** button (Ollama only) downloads a model via
    `/api/pull`. Type `gemma3:4b`, click Pull, spinner, then the
    new model lights up in the chip list. No more "switch to a
    terminal to ollama-pull" detour.

Helper text updates for the empty-model case ("Required: pick
from the list (Refresh) or type one. Use Pull to download from
Ollama.") so a fresh operator never wonders why Save / Test
errors out.

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

pubspec.yaml

@@ -1,7 +1,7 @@
 name: fai_studio
 description: "F∆I Studio — desktop GUI for the F∆I hub"
 publish_to: 'none'
-version: 0.12.1
+version: 0.13.0
 
 environment:
   sdk: ^3.11.0-200.1.beta