fix: stop the Image Gen backend probe from disabling the whole prompt form (#5671)

[atomantic]

Summary

Landing on /media/image used to grey out the entire above-the-fold form until the Stable Diffusion status probe answered — and it went dead again on every backend-chip or model switch. An unconfigured external SD API URL makes that probe time out, so the page's primary CTA was effectively unusable on first paint. ImageGen.jsx threaded statusLoading into disabled on 14 controls, including both prompt textareas and the universe/style pickers, which are pure client-side form state.

The probe decides which backend can run, not what the user may type. It now gates only:

• the backend chip strip,
• the Refresh-status button,
• the Generate submit button (plus the form's implicit-submit path).

Everything else — prompt and negative-prompt textareas, universe and style pickers, prompt helpers, LoRA/init/reference pickers, and every parameter input — stays live while the status pill is still checking. This matches VideoGen.jsx, which gates exactly one control.

Two follow-ons from local review, both shipped in this branch:

• Implicit submit. Re-enabling the form re-opens the Enter-inside-a-number-input submit path, which fires onSubmit even when the default button is disabled. handleGenerate already carried that reasoning for the edit-image / cloud-prompt hints; the probe gate now sits alongside them.
• Federated targets. A remote render dispatches to the peer, so the local probe has nothing to say about it. The gate is a single localBackendPending predicate (statusLoading && !remoteTargetActive) used by the button's disabled state, its "Checking…" label and tooltip, and the submit guard — so a hung local probe no longer blocks a render aimed at a ready peer.

While the probe is in flight the Generate button reads "Checking…" with an explanatory tooltip, so the disabled state reads as progress rather than a mystery grey button.

Test plan

New client/src/pages/ImageGen.probeGating.test.jsx (4 cases, each verified to fail against the pre-fix code for its own regression):

• prompt and negative-prompt textareas stay enabled while getImageGenStatus is pending, and Generate is disabled and labelled "Checking…";
• after the probe resolves connected: false, Generate stays disabled while both textareas stay editable;
• an implicit form submit fired during the pending probe does not reach generateImage;
• with a ready peer selected as the render target, Generate is enabled and submits even though the local probe is still hanging.

Also run:

• cd client && npm test — 854 files / 10691 tests passing.
• cd client && npm run lint — clean.
• grep -c 'disabled={statusLoading}' client/src/pages/ImageGen.jsx → 2 (acceptance criterion: at most 3).

Closes #5671

https://claude.ai/code/session_01DxNA8g7B5hZd4uswfUM1xn