fix: carry EIP-1193 error codes through to the page (closes #274)

src/content/inpage.js rebuilt every failure as `new Error(error.message)`,
so the `code` the background produced and the content script relayed intact
was dropped in the last hop. A dApp checking `err.code === 4001` — the
standard way to tell "the user said no" from "something broke" — saw
`undefined`, which makes a wallet the user deliberately declined
indistinguishable from a wallet that failed.

The provider now rejects with an EIP-1193 `ProviderRpcError` carrying the
`code` and, where the boundary sent one, `data`. A class rather than
properties attached to an `Error`: the object crosses no boundary after
construction — it is built in the page's own realm and handed straight to
the caller's catch — so the prototype survives and `name` is a stable thing
for a dApp to see.

Whatever code arrived is passed through verbatim rather than matched
against a list. The background emits 4001, 4100 and 4902 on the RPC path
today; a code added later must reach the page without editing the provider.
An error the background sent with no code stays a plain `Error` with no
`code` property at all — a `ProviderRpcError` whose `code` is `undefined`
would advertise a conformance it does not have. `message` is unchanged in
every case, and nothing the background produces changes.

Every entry point the provider exposes funnels through the one response
listener, so `request`, `enable`, `send` and `sendAsync` are all covered;
tests/inpageErrors.test.js loads the real inpage.js against a stub window
and asserts each of them, both codes and the untouched messages.

The e2e probe added for #183, which printed the missing code on all four
rejected flows rather than asserting it, now requires code 4001 on the
page's Error as well as on the wire. Without the provider change it fails
on all four with "reached the page as an error with no code property at
all".

TODO.md

@@ -45,6 +45,20 @@ undefined identifiers, which is how
 
 # Completed Steps
 
+- 2026-08-12: EIP-1193 error codes now reach the page. `src/content/inpage.js`
+  rebuilt every failure as `new Error(error.message)`, so the code the
+  background produced and the content script relayed intact was dropped in the
+  last hop and a dApp checking `err.code === 4001` saw `undefined` — a wallet
+  the user deliberately declined was indistinguishable from one that broke. The
+  provider now rejects with a `ProviderRpcError` carrying `code` and, where the
+  boundary sent one, `data`, passed through verbatim rather than matched against
+  a list, so 4001, 4100 and 4902 all arrive and a future code needs no edit
+  here. An error the background sent with no code stays a plain `Error` with no
+  `code` property, and `message` is unchanged in every case. All four request
+  entry points (`request`, `enable`, `send`, `sendAsync`) are covered by
+  `tests/inpageErrors.test.js`, and the e2e probe that printed the missing code
+  now requires it on the page's Error as well as on the wire, for all four
+  rejected flows ([#274](https://git.eeqj.de/sneak/AutistMask/issues/274)).
 - 2026-08-12: `KNOWN_SYMBOLS` now maps a symbol to the set of contract addresses
   that bear it, not to one of them. A ticker is not unique: seven of the 512
   bundled tokens — `FRAX`, `REUSD`, `TON`, `EURE`, `MSUSD`, `MUSD` and `JPYC` —