fix: move the UTC Timestamps checkbox into the Display well (closes #212)

The `settings-utc-timestamps` checkbox sat inside the Token Spam Protection
well, immediately after the dust-threshold input. It is a display preference
and has nothing to do with spam filtering, so it now renders in the Display
well next to the theme selector.

Markup move only. The checkbox is wired by id — `$("settings-utc-timestamps")`
in `src/popup/views/settings.js`, where `$` is `document.getElementById` — for
both the initial `checked` assignment and the `change` handler, so nothing in
the wiring depends on the element's parent or siblings. `state.utcTimestamps`,
`saveState`/`loadState` and the `helpers.js` formatting path are untouched.

Both Settings descriptions follow the markup: the `README.md` Screen Map and
the `docs/README.md` Settings list each now name the UTC control under Display
and no longer under Token Spam Protection.

`tests/settingsUtcTimestamps.test.js` pins both halves down: the checkbox
appears exactly once and inside the Display well rather than the spam well,
and the setting still round-trips through storage.

README.md

@@ -695,8 +695,8 @@ screen, including ExportPrivKey, falls back to Home.
       `[x]` delete button, plus a "+ Add wallet" button
     - Tracked Tokens: one row per tracked token with an `[x]` remove button,
       plus a "+ Add token" button
-    - Display: "Show tracked tokens with zero balance" checkbox and a Theme
-      selector (System / Light / Dark)
+    - Display: "Show tracked tokens with zero balance" checkbox, "UTC
+      Timestamps" checkbox, and a Theme selector (System / Light / Dark)
     - Network: network selector (Ethereum Mainnet / Sepolia Testnet); switching
       resets the RPC and Blockscout endpoints to that network's defaults
     - Ethereum RPC: endpoint URL input + "Save" button (validated against
@@ -707,7 +707,6 @@ screen, including ExportPrivKey, falls back to Home.
         - "Hide tokens with fewer than 1,000 holders" checkbox
         - "Hide transactions from detected fraud contracts" checkbox
         - "Hide dust transactions below N gwei" checkbox + threshold input
-        - "UTC Timestamps" checkbox
     - Allowed Sites: list with remove buttons
     - Denied Sites: list with remove buttons
     - About: project link, license, author, version, release date, and the