test: drive ConfirmTx in the e2e suite, gate assertion included (closes #238)

ConfirmTx -- the screen that decides what gets signed -- had no automated
coverage of its own behaviour. The arithmetic underneath was well tested; the
wiring was not, so a mutant making the spend gate read the displayed fee
estimate instead of the reserve would have reintroduced the #154 overspend with
the suite still green.

Nine end-to-end tests now drive it for both the native and ERC-20 paths,
covering the pending, funded, over-balance and estimate-failed states, and
asserting that the gate reads the reserve rather than the estimate. Swapping the
two makes the suite fail. The view height is asserted constant across every
state transition rather than merely printed.

Reaching the screen needs a funded balance and a gas estimate, so the route
interception gains fixtures for both. Testing the estimate-failed state means
provoking the console error the code is supposed to emit, which the harness
otherwise fails a run on; an expectation mechanism consumes exactly one matching
record, is scoped to the declaring test, and fails that test if nothing matched,
so it cannot mask an unrelated error.

TODO.md

@@ -44,6 +44,14 @@ undefined identifiers, which is how
 
 # Completed Steps
 
+- 2026-08-12: The transaction confirmation screen has browser coverage. The
+  end-to-end suite reaches ConfirmTx for both the native ETH and the ERC-20 path
+  off a funded-balance fixture, and asserts the pending, funded, over-balance
+  and estimate-failed states, the fee block quoting the estimate and the reserve
+  separately, and a constant view height across every one of those transitions.
+  The load-bearing assertion is that the spend gate reads the reserve and not
+  the displayed estimate: swapping the two fails the suite
+  ([#238](https://git.eeqj.de/sneak/AutistMask/issues/238)).
 - 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