fix: explain a rejected dust threshold instead of silently snapping back (closes #233)

The dust-threshold field was the only validated input in Settings that rejected
without saying anything: the value silently changed back to the stored one with
no explanation. It now flashes "Please enter a whole number of gwei, zero or
greater." alongside the existing resync, matching the idiom the RPC URL field
already uses.

The parse moves to its own module and accepts plain decimal digits only, zero
or greater. Hex and exponent notation are refused rather than accepted: Number()
reads "0x10" as 16 and "1e3" as 1000, neither of which the previous parseInt
produced, and storing a number the user did not type is the same silent
substitution this change exists to remove.

The message must fit one line of the reserved flash area -- a wrapped message
pushes the settings view down, which the No Layout Shift policy forbids. That is
pinned by an end-to-end test measuring the rendered line height and the position
of the elements below it, in a single round trip because the flash clears after
two seconds.

src/popup/views/settings.js

@@ -9,6 +9,10 @@ const {
     pushCurrentView,
 } = require("./helpers");
 const { applyTheme } = require("../theme");
+const {
+    DUST_THRESHOLD_MESSAGE,
+    parseDustThresholdGwei,
+} = require("../dustThreshold");
 const { state, saveState, currentNetwork } = require("../../shared/state");
 const { NETWORKS, SUPPORTED_CHAIN_IDS } = require("../../shared/networks");
 const { onChainSwitch } = require("../../shared/chainSwitch");
@@ -329,13 +333,14 @@ function init(ctx) {
 
     $("settings-dust-threshold").value = state.dustThresholdGwei;
     $("settings-dust-threshold").addEventListener("change", async () => {
-        const raw = $("settings-dust-threshold").value.trim();
-        const val = Number(raw);
-        // 0 is accepted and means "hide nothing".  Empty, negative,
-        // fractional and non-numeric input is rejected outright rather than
-        // coerced, and the field is put back to the stored threshold so it
-        // never shows a value the wallet is not using.
-        if (raw !== "" && Number.isInteger(val) && val >= 0) {
+        const val = parseDustThresholdGwei($("settings-dust-threshold").value);
+        // Rejected input is never coerced. The field is put back to the
+        // stored threshold so it never shows a value the wallet is not
+        // using, and the message says what the field wants so the snap-back
+        // is explained rather than silent.
+        if (val === null) {
+            showFlash(DUST_THRESHOLD_MESSAGE);
+        } else {
             state.dustThresholdGwei = val;
             await saveState();
         }