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>
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