fix: Zero ERC-20 plugin context and require both ABI fields before review

The ERC-20 internal plugin only cleared `extra_data` at
ETH_PLUGIN_INIT_CONTRACT, leaving `destinationAddress`, `amount`,
`ticker`, `decimals`, `contract_name`, and `selectorIndex` from the
previous signing flow alive in the per-plugin global context that
backs dataContext.tokenContext.pluginContext. A host that started a
new transfer with selector-compatible but malformed calldata (wrong
ABI offsets, truncated payload) could then reach
ETH_PLUGIN_FINALIZE with neither the destination nor the amount
parameter actually observed, and the trusted review screen would
display the stale values from the previous flow as if they were the
current ones.

Zero the entire context at INIT and add `destination_parsed` /
`amount_parsed` flags that the parameter handlers flip when the
two mandatory ABI offsets fire. FINALIZE refuses the screen if
either flag is missing, so non-conforming calldata is reported as
ERROR instead of rendering stale state.

(cherry picked from commit 816010e482302852b097a3089299c3f6705e00c3)