A save wrote `trackedTokens` whole, so two pages on one network dropped each
other's adds and brought back each other's removals.
A save that keeps the network now merges this page's change into the stored
list by contract address. A save that switches networks is unchanged.
`mergeTrackedTokens()` does that merge, and the two other merges of tracked
tokens now call it. It keeps one record per contract address, this page's for
a token both lists hold, so a token held twice no longer multiplies.
Model: opus-5-5
sneak