test image-gen connection against the provider tab you're on, not the saved default

[atomantic]

Summary

Pressing Test Connection on the Grok CLI or Agy CLI tab of Settings → Image Gen reported codex — codex-cli 0.145.0, which reads as though Agy/Grok had resolved to the codex binary.

Nothing was wrong with those probes — they were never run. The button lives in the global save/status bar shared by every sub-tab and called /api/image-gen/status with no ?mode=, so imageGen.checkConnection() fell back to settings.imageGen.mode (the saved default backend) and answered for that instead.

Changes, all in client/src/components/settings/ImageGenTab.jsx:

• Each provider tab in MEDIA_TABS carries a probeMode, passed to getImageGenStatus(mode). The four non-provider tabs (Backend / Tokens / Expose / Test) carry none and keep probing the saved default.
• Every result is tagged with the tab that requested it and rendered only under that tab, so a probe left in flight across a tab switch can't paint its answer under another provider's panel.
• The button tooltip names the backend it will probe.

A probe of a disabled provider now truthfully reports "… is disabled in settings" instead of a false green borrowed from whatever was saved as default. No server change — /status already validated and honored ?mode=.

Follow-up captured in PLAN.md: the Test tab's Test Render still renders with the saved default mode, so smoke-testing an Agy render still requires making it the default. That needs a provider picker, not a re-target of an existing call.

Test plan

• client/src/components/settings/ImageGenTab.test.jsx — five new cases: per-tab probe targeting (agy, grok), fallback to the default on non-provider tabs, result hidden after a tab switch, a deferred-promise case proving a late response never surfaces under the tab the user switched to, and an exhaustiveness assertion that every IMAGE_GEN_MODE is tagged onto some tab (so a future backend can't silently regress to probing the default).
• Full client suite green: 476 files / 5326 tests.
• ESLint clean on both changed files.