fix: a save merges this page's tracked token change into the stored list (closes #717)

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 token both lists hold keeps this page's record.
Two pages that add one token can hold different records for it, and
`mergeListByIdentity()` alone would store both.

A save that switches networks is unchanged.

Model: opus-5-5

README.md

@@ -397,10 +397,16 @@ is missing, a write fails.
 
 The popup's `saveState()` (`src/shared/state.js`) reads the stored record,
 merges this page's changes into it and writes it back. It writes only the fields
-this page changed since its last save. Wallets, site lists and endpoints are
-merged entry by entry, so two pages changing different things do not overwrite
-each other. The background's `updateState()` reads the record, applies one
-change and writes it back.
+this page changed since its last save. It merges the fields below entry by
+entry, so two pages changing different things do not overwrite each other:
+
+- wallets;
+- site lists;
+- endpoints;
+- tracked tokens.
+
+The background's `updateState()` reads the record, applies one change and writes
+it back.
 
 Every write stamps `schemaVersion` with `STATE_SCHEMA_VERSION` from
 `src/shared/stateSchema.js`, currently `1`. The version is raised only when the