fix: one transaction approval at a time, and honest copy for a nonce collision (closes #271)

TODO.md

@@ -70,6 +70,31 @@ undefined identifiers, which is how
   stubbed RPC and the unobservable toolbar popup — were checked against the
   current `src/content/inpage.js` and `tests/e2e/` and left as they are
   ([#285](https://git.eeqj.de/sneak/AutistMask/issues/285)).
+- 2026-08-17: One transaction approval at a time. Populating in the background
+  before the window opens is what makes the displayed object the verified
+  object, and it also fixes the nonce: two `eth_sendTransaction` calls populated
+  concurrently took the same nonce from a node that had seen neither broadcast,
+  and the second could then never be sent, because the only way to give it a
+  fresh nonce is to populate it again after the user has read the old one off
+  the screen. A second request is now refused with EIP-1193 `-32002` while one
+  is unanswered — the slot is taken immediately before population, after the
+  authorization checks, so no second nonce is allocated, no second window opens,
+  and a page the wallet refuses anyway cannot hold the slot against the
+  connected site. The slot is freed at `settleApproval()`, the single point an
+  approval is retired, so every path that ends an approval ends the hold with
+  it; an approval whose window is gone and whose attempt has failed is settled
+  there rather than left waiting on a window that no longer exists, and an
+  approval window that could not be opened at all is answered with `-32603`
+  instead of holding the page's promise open. Signature approvals are not gated,
+  consuming no nonce. A collision that does happen is also reported accurately
+  now: a broadcast the node refused for the nonce, and an approval carrying a
+  nonce this worker has already broadcast for that address on that chain (caught
+  before the node is asked at all), both say the transaction did not reach the
+  network and to send it again, instead of warning that it may have sent. The
+  record is keyed by chain as well as address, because nonce spaces are per
+  chain and low nonces overlap across them. `already known` deliberately keeps
+  the ambiguous wording, because a node that says it has the transaction has it
+  ([#271](https://git.eeqj.de/sneak/AutistMask/issues/271)).
 - 2026-08-14: The parts of the
   [#150](https://git.eeqj.de/sneak/AutistMask/issues/150) and
   [#151](https://git.eeqj.de/sneak/AutistMask/issues/151) definition of done the