refactor: one shared extension-API module, and drive the dApp flows on Firefox (closes #153)

TODO.md

@@ -59,6 +59,24 @@ undefined identifiers, which is how
   screen's container holds at 20px with the following section at the same offset
   for the old string, the new string and the empty reserved state
   ([#265](https://git.eeqj.de/sneak/AutistMask/issues/265)).
+- 2026-08-17: One shared extension-API module,
+  [`src/shared/browserApi.js`](src/shared/browserApi.js), is the only place in
+  the tree that names `browser` or `chrome`. Every call site returns a promise;
+  `runtime.lastError` is gone. The same commit gives the Firefox suite the four
+  dApp round trips — `eth_requestAccounts`, `personal_sign`,
+  `eth_sendTransaction` and a closed approval window rejecting with EIP-1193
+  4001 — against a page and a JSON-RPC node served from loopback, which survives
+  `--network none`. **The premise of
+  [#153](https://git.eeqj.de/sneak/AutistMask/issues/153) does not survive that
+  harness**: Firefox's `browser.*` honours a trailing Chrome-style callback and
+  populates `runtime.lastError`, both measured directly on Firefox 153.0.3, and
+  all four flows pass against the unconverted code. What landed is a uniformity
+  and coverage change, not a repair of a broken target. `storageGet()` and
+  `storageSet()` **reject** where `storage.local` is absent rather than
+  resolving `{}` and a no-op write — they carry the wallet, and defaulting would
+  read an existing wallet back as none. The one caller that genuinely degrades,
+  [`src/shared/phishingDomains.js`](src/shared/phishingDomains.js), takes
+  `storageLocal()` directly and keeps its own null check.
 - 2026-08-17: An address total no longer reports `$0.00` for holdings it cannot
   price. Prices exist for the top 25 tokens only, so the priced-only sum was
   printed as the total and an address holding nothing but unpriced ERC-20s was