🐛 fix(webui): support model removal and force dispatch

[gubaiovo]

Summary

• rebind global Solver, TLDR, Father, and SubAgent defaults when a model is renamed or removed
• reject global model deletion with a concrete 409 response while a loaded contest still references it
• add a confirmation-backed 强制派发 action for solved challenges and send force: true explicitly
• cover both flows with route and component behavior tests

Validation

• corepack pnpm exec vp check (0 errors; 1 pre-existing packages/shellpoop warning)
• corepack pnpm exec vp run typecheck
• corepack pnpm exec vp test run apps/backend/test/models.test.ts apps/frontend/test/model-pool-editor.test.ts apps/frontend/test/challenge-details.test.ts (19 passed)

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