[FIX] Normalize free OpenCode model selection during onboarding

[vmanvs]

Summary

OpenCode providers may return models either as arrays or as object maps keyed by model ID. Previously, free-model selection only handled array responses and checked only model display names for the free marker.

As a result, onboarding could miss an available free model and fall back to the first model returned by the provider. This PR applies the same response-shape handling already established in AgentForm.tsx, while additionally checking both model IDs and display names to select a free default.

Changes

• Support both array and object-map model responses when selecting a free OpenCode model.
• Detect the free marker in both model IDs and display names.
• Use explicit model IDs when present, falling back to object-map keys.
• Add regression tests for all supported response shapes and fallback behavior.

Testing

• pnpm typecheck passes.
• pnpm build succeeds.
• Focused onboarding test suite passes: 27 tests.
• Ran the Windows-equivalent full test suite:

$env:ELECTRON_RUN_AS_NODE='1'
.\node_modules\.bin\electron.cmd .\node_modules\vitest\vitest.mjs run

Checklist

• pnpm typecheck passes
• pnpm build succeeds
• pnpm test:run passes
• No hardcoded color classes (use CSS variable tokens)

Notes

The full suite completed with unrelated existing failures:

Test Files  3 failed | 94 passed (97)
Tests       11 failed | 1438 passed (1449)

Failures remain in agent-manager.test.ts, ipc-handlers.test.ts, and opencode-config.test.ts.
Literal pnpm test:run is not Windows-compatible because it uses Unix-style environment-variable syntax.

[vmanvs]

@dimavedenyapin please review these changes