fix: union the authoritative vision-model list into VisionProviderPicker (#4157)

[atomantic]

Summary

VisionProviderPicker (the universe builder's image-describe, style-reference and corrective-reference pickers) filtered local backends with the client-side id regex alone, so an installed VLM from a family the regex predates was hidden — the picker showed a "no vision-capable model installed" blocker next to an empty dropdown. Every other vision picker (AiAssignmentsTab, CreativeDirectorModelsDrawer, PromptFromMedia) already unions the server's authoritative per-provider set from useVisionModelIds; this one now passes it as visionLocalModelFilter's third argument too.

That set resolves after the first provider load, so the modelFilter's identity changes — which useProviderModels handled badly in two ways, both fixed here:

• load depended on pickInitialModel (and so on modelFilter), so a filter change re-ran the whole api.getProviders() fetch for data it already had. The picker now reaches the async body through a ref, keeping load's identity stable.
• hasSetInitialRef froze the auto-pick at whatever the first, blinder filter produced, so the authoritative list never got a say — selectedModel stayed '' while availableModels filled in. A new effect re-runs the initial pick when the filter's identity changes and the current selection is no longer in the list. It stands down permanently once the user picks or clears a model: '' alone can't distinguish a deliberate clear from "the filter matched nothing", so a userPickedModelRef latch carries that distinction. Scoped to filtered, non-allowDefault pickers, so the ~25 other consumers of the hook are untouched.

The picker also holds its "no vision model" blocker until the capability scan settles on a local backend (the visionPending idiom already used in StackerNews / CreativeDirectorModelsDrawer), so it no longer flashes the blocker mid-fetch, and it reports that window as loading to its callers.

PromptFromMedia passes a filter with the same changing identity and silently inherits both fixes.

Test plan

• client/src/hooks/useProviderModels.test.js — new block for a modelFilter whose identity changes: re-picks once the filter widens; no second getProviders() call; leaves a deliberate clear and a user-picked model alone; re-arms after a provider change; allowDefault still keeps the empty-model sentinel.
• client/src/components/universe/VisionProviderPicker.test.jsx — new file (the component had none): offers a VLM the id regex doesn't recognize once the server list lands, still blocks when the backend genuinely has none, doesn't flash the blocker mid-scan, leaves a cloud provider unfiltered, and explains the no-provider case.
• Both suites were verified to FAIL against the unfixed source (3 hook tests, 2 picker tests) before the fix was restored.
• VisionDescribeModal.test.jsx stubs useVisionModelIds — the picker now runs a capability scan on mount, which the suite would otherwise hit outside act().
• Full client suite: cd client && npx vitest run → 646 files, 7877 tests passing (one unrelated QuotaBurn.test.jsx parallel-run flake, green in isolation).
• npx biome lint --error-on-warnings clean on every touched file.

Closes #4157