Fix(context): clear dataContext in reset_app_context()

tokenContext_t (plugin name, contract address, method selector,
pluginChainId, pluginStatus, token lookups) was never zeroed between
signing sessions, allowing stale plugin/token state from a previous
transaction to bleed into the next one.

Add memset(&dataContext) alongside the existing clears of tmpCtx,
txContext and tmpContent.

(cherry picked from commit 68224b24cc3cdaedfa2104e495be9baf52e66b7f)