dot/state: `headerHashKey` should accept a `uint` instead of `uint64`

## Task summary

- `encodeBlockNumber` should be changed to encode a `uint` to a dynamically sized byte slice using `common.UintToBytes` to save space
- `headerHashKey` should accept a `uint` instead of `uint64`
- This will break existing database storage, so we should definitely do this before a v1 release.

Locate encodeBlockNumber and headerHashKey in the dot/state implementation, then inspect their callers and database-related tests. Read common.UintToBytes and verify how encoded block numbers are stored and retrieved. Done means both functions use uint consistently, dynamic encoding is covered by tests, and the intentional pre-v1 storage break is understood.

Written by the indexing model from the issue text.