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.

README.md

@@ -857,7 +857,12 @@ on ConfirmTx, DeleteWallet, ApproveTx and ApproveSign.
         - "Hide fake tokens impersonating a known symbol" checkbox
         - "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
+        - "Hide dust transactions below N gwei" checkbox + threshold input. The
+          threshold is plain decimal digits, a whole number of gwei, zero or
+          greater (zero hides nothing). Anything else — a fraction, a negative,
+          a value carrying its unit, hex (`0x10`) or exponent (`1e3`) notation —
+          is refused with a flash message and the field snaps back to the stored
+          threshold, so a number the user did not type is never stored.
     - Allowed Sites: list with remove buttons
     - Denied Sites: list with remove buttons
     - About: project link, license, author, version, release date, and the