fix: one transaction history row per value movement (closes #177)

A plain ERC-20 transfer produced two rows: the token-transfer row and the
zero-ETH native row for the same hash. parseTx leaves method "transfer" out
of the display-level contract-call case, so the merge loop's
direction === "contract" test never absorbed the native side.

The merge is now the pure mergeTransactions(txs, tokenTransfers) in
src/shared/transactions.js, unit tested directly. It keys the native entry
by hash and each token transfer by hash plus token contract, and drops the
native entry when it moved no ETH and a token transfer shares its hash. A
native entry that moved ETH survives beside the token rows, a zero-value
native transaction with no token transfer on its hash still displays, and a
display-level contract call keeps consolidating its legs into one row.

The dust filter's isContractCall exemption is unchanged: it still carries
approve and other zero-ETH calls that have no token row to be represented
by.

TODO.md

@@ -44,6 +44,10 @@ undefined identifiers, which is how
 
 # Completed Steps
 
+- 2026-08-11: One row per on-chain value movement in transaction history: the
+  merge moved into the pure `mergeTransactions` and the zero-ETH native side of
+  a plain ERC-20 transfer absorbed into its token row
+  ([#177](https://git.eeqj.de/sneak/AutistMask/issues/177)).
 - 2026-08-11: `TODO.md` Workflow rewritten to the branch-and-PR-per-issue model
   on `next`, with Status and Next Step refreshed
   ([#191](https://git.eeqj.de/sneak/AutistMask/issues/191)).