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

Every call site that touched `browser.*` or `chrome.*` now goes through
`src/shared/browserApi.js`, the only file in the tree that names either.
It exposes lazily-resolved namespace handles for events and synchronous
methods, and promise-returning wrappers for everything that is
callback-shaped on Chrome. Callers await; `runtime.lastError` is gone,
folded into the rejection the wrapper produces on the Chrome path.

The Firefox suite gains the four dApp round trips the issue's definition
of done asks for — `eth_requestAccounts`, `personal_sign`,
`eth_sendTransaction`, and a closed approval window rejecting with
EIP-1193 4001 — driven through the real content script, background page
and approval windows. `--network none` was thought to rule that out
because it leaves no `http://` origin to inject into; loopback survives
it, so the page and a JSON-RPC node are served from 127.0.0.1 inside the
container and the run still reaches nothing but itself.

That harness refutes the premise it was built to verify. On Firefox
153.0.3, `browser.*` honours a trailing Chrome-style callback and does
populate `runtime.lastError`, both measured directly, and all four flows
pass against the unconverted code. So this is a uniformity and coverage
change, not a repair of a broken target; the PR records the measurement
in full.

One real defect is fixed on the way past: the window id written back
into a pending approval after `windows.create()` was unguarded, so an
approval settled during the open — an address switch will do it —
dereferenced a deleted entry.

tests/e2e/firefox/driver.js

@@ -110,6 +110,26 @@ class Driver {
             "extensions.webextensions.uuids": JSON.stringify({
                 [EXTENSION_ID]: EXTENSION_UUID,
             }),
+            // The container has loopback and nothing else. Firefox's own
+            // link-status detection can read that as "offline" and then
+            // refuse every request, including the ones to the loopback dApp
+            // origin the suite serves; this takes the decision away from it.
+            "network.manage-offline-status": false,
+            // Force the site-connection prompt down its windows.create()
+            // fallback.
+            //
+            // src/background/index.js prefers the toolbar-anchored popup for
+            // that one approval and opens a real window only when
+            // openPopup() refuses. A panel is not a top-level browsing
+            // context, so WebDriver cannot see it, list it or click in it —
+            // the same blind spot the Chrome harness documents. Leaving this
+            // at its default would make which path runs depend on whether a
+            // headless Firefox counts as having had a user gesture, which is
+            // not a thing to leave to chance in a suite that has to be able
+            // to fail. The window path is shipped code and the same approval
+            // id, so what is driven is real; what is NOT covered either way
+            // is the panel presentation itself.
+            "extensions.openPopupWithoutUserGesture.enabled": false,
         };
 
         const value = await this.send("POST", "/session", {
@@ -179,6 +199,16 @@ class Driver {
         return this.session("POST", "/execute/sync", { script, args });
     }
 
+    // The asynchronous form: the script is handed a resolve callback as its
+    // last argument and the call settles when that is invoked. Everything
+    // interesting about an extension page is promise-shaped — storage reads,
+    // the provider's own request() — and /execute/sync cannot wait for any
+    // of it.
+    async executeAsync(script, args = []) {
+        await this.setContext("content");
+        return this.session("POST", "/execute/async", { script, args });
+    }
+
     // Runs in the privileged chrome scope, where Services and Ci exist.
     async executeChrome(script, args = []) {
         await this.setContext("chrome");
@@ -329,6 +359,63 @@ class Driver {
             [selector],
         );
     }
+
+    // ------------------------------------------------------------ windows
+    //
+    // The approval prompts this suite drives are separate top-level windows
+    // the extension opens itself, so every one of them is a window handle
+    // here and the suite has to move between them explicitly.
+
+    async windowHandles() {
+        return this.session("GET", "/window/handles");
+    }
+
+    async currentWindow() {
+        return this.session("GET", "/window");
+    }
+
+    async switchToWindow(handle) {
+        await this.setContext("content");
+        await this.session("POST", "/window", { handle });
+    }
+
+    async newWindow(type = "window") {
+        await this.setContext("content");
+        const value = await this.session("POST", "/window/new", { type });
+        return value.handle;
+    }
+
+    // Closes the current window and leaves the session on `fallback`, because
+    // a session whose current window is gone fails every subsequent command
+    // with "no such window" rather than with anything diagnosable.
+    async closeWindow(fallback) {
+        await this.setContext("content");
+        await this.session("DELETE", "/window");
+        if (fallback) await this.switchToWindow(fallback);
+    }
+
+    async url() {
+        return this.session("GET", "/url");
+    }
+
+    // The handle of the first window whose URL matches, or null. Restores the
+    // window that was current before the search either way: a probe that
+    // silently relocates the session is a trap for the step after it.
+    async findWindow(predicate) {
+        const origin = await this.currentWindow();
+        try {
+            for (const handle of await this.windowHandles()) {
+                await this.switchToWindow(handle);
+                if (predicate(await this.url())) return handle;
+            }
+            return null;
+        } finally {
+            // Tolerated: the window the search started from may have been the
+            // one that just closed, and a throw in here would replace the
+            // real result with "no such window".
+            await this.switchToWindow(origin).catch(() => {});
+        }
+    }
 }
 
 // ------------------------------------------------------- error capture
@@ -349,10 +436,15 @@ class Driver {
 // background page, which BiDi would not have covered even if it worked.
 // Background-page capture is verified by probe — a throw at the top of
 // src/background/index.js, which kills the background page outright, fails
-// the run. Content-script errors should arrive by the same route, but that
-// is UNVERIFIED here and must not be claimed: the container runs with
-// --network none, so there is no http:// page for a content script to be
-// injected into and this suite never exercises one.
+// the run.
+//
+// Content scripts ARE now exercised: tests/e2e/firefox/dapp.js serves a page
+// from loopback, which survives --network none, and the suite drives the
+// EIP-1193 round trips through the content script injected into it. What is
+// still unproven is the CAPTURE, not the execution — no probe has forced a
+// throw from inside a content script and watched it fail the run, so an
+// uncaught content-script error arriving by this route remains an
+// expectation rather than a demonstrated fact. Do not claim otherwise.
 //
 // Warnings are excluded so the semantics match Playwright's pageerror:
 // uncaught errors only.