feat(registry): surface legacy registry format with targeted UI

[peppescg]

Summary

When the configured custom registry serves data in the legacy ToolHive format, today the user sees the generic "Oops, something went wrong" error boundary even though the backend (stacklok/toolhive#5260) returns a structured HTTP 502 Bad Gateway with code: "registry_legacy_format". This PR wires the renderer to that contract so the user is redirected to Settings > Registry with a clear toast, just like the existing registry_auth_required / registry_unavailable flows.

Closes #2228
Depends on stacklok/toolhive#5260

Why this matters

Real-world report (#user-chat, May 12, 2026): after upgrading Studio to 0.34, the app failed to start because the previously-saved custom registry URL was returning legacy JSON. Recovery required CLI debugging and thv config unset-registry. The actionable hint is already in the API response — the renderer just needed to read it.

Changes

• registry-errors-message.ts — new REGISTRY_LEGACY_FORMAT_CODE constant, isRegistryLegacyFormatError detector, REGISTRY_LEGACY_FORMAT_UI_MESSAGE + REGISTRY_LEGACY_FORMAT_TOAST_MESSAGE (no raw thv registry convert hint — desktop users should not see CLI commands), and an extension of getRegistryErrorToastMessage so the root beforeLoad guard already redirects to Settings on this code.
• registry-error.tsx — third empty-state branch ("Unsupported registry format") for users who navigate directly to /registry while the source is misconfigured.
• __tests__/registry-error.test.tsx — three new cases covering the legacy-format branch, including an explicit assertion that the raw CLI hint never reaches the visible UI.
• __tests__/handle-registry-auth-redirect.test.ts — verifies the guard redirects to /settings?tab=registry with the legacy-format toast when the API returns the new code.

Test plan

• pnpm run test:nonInteractive on the touched files — 23 tests pass
• pnpm run type-check clean
• pnpm run lint clean
• Manual E2E against a backend serving legacy JSON: registry GET returns 502 + code: registry_legacy_format → Studio redirects to Settings > Registry with the toast, no error boundary

Reproducing locally

A backend in the failing state is needed. Until stacklok/toolhive#5260 merges:

```bash

Build backend from PR branch

cd && git checkout issues/5259
go build -o /tmp/thv-fix ./cmd/thv

Isolate config and point to a legacy JSON file

mkdir -p /tmp/thv-fix-test/toolhive
cat > /tmp/legacy-registry.json <<'JSON'
{"version":"1.0.0","servers":{"demo":{"image":"demo:latest","tier":"Community","transport":"stdio"}}}
JSON
cat > /tmp/thv-fix-test/toolhive/config.yaml <<'YAML'
secrets: { provider_type: "", setup_completed: false }
clients: { registered_clients: [] }
local_registry_path: /tmp/legacy-registry.json
YAML

Run backend

XDG_CONFIG_HOME=/tmp/thv-fix-test TOOLHIVE_SKIP_DESKTOP_CHECK=1 /tmp/thv-fix serve --port 8181

Run Studio dev from this worktree, attached to the backend

THV_PORT=8181 pnpm run start
```

To restore once verified:
```bash
curl -s -X PUT http://localhost:8181/api/v1beta/registry/default -H 'Content-Type: application/json' -d '{}'
```

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR updates the renderer to recognize the backend’s structured code: "registry_legacy_format" error and handle it with the same targeted redirect/toast UX used for other registry misconfiguration errors, avoiding the generic error boundary.

Changes:

• Add detection and dedicated toast/UI copy for registry_legacy_format, and include it in the startup redirect guard toast mapping.
• Add a dedicated “Unsupported registry format” empty-state branch on the Registry error page.
• Extend unit tests to cover the new legacy-format redirect and ensure CLI hints from raw API messages are not displayed.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
renderer/src/routes/root/guards/tests/handle-registry-auth-redirect.test.ts	Adds coverage that the guard redirects + stores the legacy-format toast when the API returns registry_legacy_format.
renderer/src/common/components/settings/registry/registry-errors-message.ts	Introduces legacy-format error code detection and user-facing toast/UI messages, and wires it into getRegistryErrorToastMessage.
renderer/src/common/components/error/registry-error.tsx	Adds a dedicated empty state for unsupported legacy registry format with Settings CTA.
renderer/src/common/components/error/tests/registry-error.test.tsx	Adds tests for the legacy-format empty state and asserts raw CLI hints aren’t rendered.

[samuv]

🚀