fix: warn the user when undelegating is blocked by a pending delegator

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Switching delegate sets `startRound` to `currentRound + 1`, making the delegator **Pending** for the rest of the round. `BondingManager` rejects `unbond` in that state, so the simulation reverts and `Undelegate.tsx:86` disables the button — correctly, but with no explanation.

`renderUnstakeWarnings` in `Footer.tsx` already has the right message:

> Your account is in a pending state. You can undelegate during the next round.

It never renders because `getDelegatorStatus` (`lib/utils.tsx:30-57`) compares **subgraph** values. After a switch the new `startRound` is not yet indexed, so it returns `Bonded` while the chain says `Pending`.

Observed: on-chain `startRound` 4276 against `currentRound` 4275 — genuinely pending, no notice shown.

## Reproduction

1. Delegate to orchestrator A.
2. Move Delegated Stake to orchestrator B.
3. Same round, Undelegate tab, enter an amount below your stake.
4. Button is disabled with no reason. Clears when the next round starts.

## Fix

Read `BondingManager.delegatorStatus(address)` on-chain so the pending warning shows. Sourcing only `currentRound` on-chain is not enough — the stale value is `startRound`, so both operands must come from the chain.

Related: livepeer/explorer#739.

Start with `lib/utils.tsx:30-57`, `Footer.tsx` and `Undelegate.tsx:86`, then trace how `BondingManager.delegatorStatus(address)` is read. Reproduce the same-round undelegation scenario and verify that the pending warning renders when the on-chain status differs from stale subgraph values.

Written by the indexing model from the issue text.