feat(staking): warn on empty reward-pool contracts [OPE-1846]

[Tanya-atatakai]

Context

Ticket #1175 / OPE-1846: an Omenstrat agent (service 3768) was deployed and actively trading but showed no staking status/history. Root cause: the target contract pearl_beta_mech_marketplace_1 had an empty reward pool (availableRewards == 0), so the middleware silently omitted the stake from the deploy mega-batch (WARNING only, deploy proceeds) — the service runs and trades but is unstaked, and the app rendered a blank staking panel with no explanation.

The frontend already computed isRewardsAvailable / hasEnoughRewardsAndSlots but never used them: users could select or stay on a rewards-exhausted contract with zero signal. Slots were gated everywhere; rewards were not.

Changes (non-blocking warnings — no gating behavior changed)

• New shared NoStakingRewardsAlert (type="warning"): "No staking rewards available — reward pool is currently empty; the agent can still run but won't earn until it's refilled." Optional Switch Staking Contract action.
• Migration / onboarding (SelectActivityRewardsConfiguration): per-card warning when a contract's reward pool is empty. Card stays selectable — useCanMigrate is untouched.
• Main page (AgentDisabledAlert): warning shown when the selected contract has availableRewards === 0, rendered alongside the self-hiding low-balance alerts so neither is masked.

Gating reads loaded contract details (availableRewards === 0 / stakingContractInfo && !isRewardsAvailable), not raw isRewardsAvailable, to avoid flashing during load or firing on no_staking (query disabled → undefined).

Tests

• NoStakingRewardsAlert — message render + optional switch button.
• StakingRewardsWarning — load / rewards-available / empty-pool gating.
• Existing SelectStakingPage + AgentInfo suites pass (72). lint:frontend 0 errors, typecheck clean.

Out of scope / notes

• Middleware unchanged (per request): it still silently omits the stake when rewards are empty. This PR only surfaces the condition in the UI.
• Warning also shows during onboarding (shared card), not just migration — it's the exact path that produced the unstaked service 3768. Easy to restrict to migrate-only if preferred.
• AgentDisabledAlert/index.tsx has no unit-test harness (only its sub-alerts are tested in isolation); the equivalent gating pattern is covered by the StakingRewardsWarning test.

Screenshots

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