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.

TODO.md

@@ -44,6 +44,11 @@ undefined identifiers, which is how
 
 # Completed Steps
 
+- 2026-08-12: The dust threshold field now explains a rejection instead of
+  snapping back in silence, with the parse in a pure, unit-tested module that
+  accepts plain decimal digits only — hex and exponent notation are refused
+  rather than read as 16 and 1000
+  ([#233](https://git.eeqj.de/sneak/AutistMask/issues/233)).
 - 2026-08-12: Approval verification became an allowlist — transaction type
   restricted to 0/1/2 so an EIP-7702 delegation can no longer ride along on an
   approved transfer, every consequential field compared, the artifact