fix: run libsodium on WebAssembly under the extension CSP (closes #182)

libsodium ships a WASM build and a wasm2js translation in one file, tries
WASM first, and silently falls back if instantiation throws. Under a plain
script-src 'self' the fallback was taken on every popup load, announced by
nothing but an uncaught CompileError.

Measured on the vault's own Argon2id parameters (OPSLIMIT_INTERACTIVE,
MEMLIMIT_INTERACTIVE), node 22: WASM 141-198ms per derivation, wasm2js
3204-3660ms. The work factor is identical either way — it is set by the
ops and memory parameters, not by wall time — so the fallback bought no
security and cost about 3.5s on every operation that asks for the
password, which is every signature.

Both manifests now declare script-src 'self' 'wasm-unsafe-eval';
object-src 'self' for extension pages: an object under
content_security_policy.extension_pages for Chrome MV3, a bare string for
Firefox MV2. The keyword permits compiling WebAssembly and nothing else —
not eval() of strings, not inline script, not remote script — and reaching
it requires already executing script in an extension page. 'unsafe-eval'
is not granted.

The silence is what made this dangerous, so the fallback is now loud at
three levels: tests/manifest.test.js pins both policies to exactly that
token set, failing make check if the grant is dropped or if anything is
added beside it; tests/vaultBackend.test.js asserts the unit tests
exercise the WASM backend, with a self-validating check that libsodium
never swapped its fallback in; and the e2e suite compiles a WebAssembly
module inside the real popup under the real manifest, with the harness
allowlist entry that used to excuse the CompileError now deleted.

The runtime fallback itself is kept — a wallet that refuses to decrypt is
worse than a slow one — but vault.js now reports the backend and logs an
error when it is not WASM.

tests/e2e/run.js

@@ -14,6 +14,7 @@ const {
     launch,
     openAddressDetail,
     openPopup,
+    pageCompilesWasm,
     visible,
 } = require("./harness");
 const { STUB_TOKEN, STUB_TX_HASH } = require("./network");
@@ -55,6 +56,27 @@ test("popup loads and reaches the welcome view", async (env) => {
     assert(title === "AutistMask", "unexpected popup title: " + title);
 });
 
+// The empirical half of #182. The manifest change is only a claim about
+// what the CSP permits; this is the observation. Two things have to hold
+// together, and the run covers both: the popup realm compiles WASM (here),
+// and no WASM refusal or abort is recorded anywhere in the run — the
+// harness allowlist that used to excuse exactly that error is now empty,
+// so a recurrence fails whichever test it lands in rather than being
+// tolerated. Since libsodium's WASM module is embedded in the bundle and
+// needs no fetch, a realm that compiles WASM is a realm where libsodium
+// takes the WASM path, and the next test drives a real vault encryption
+// through it.
+test("the popup compiles WebAssembly under the shipped CSP (#182)", async (env) => {
+    const ok = await pageCompilesWasm(env.page);
+    assert(
+        ok,
+        "the popup refused to compile WebAssembly. The shipped manifest CSP " +
+            "has lost 'wasm-unsafe-eval', so libsodium is back on its wasm2js " +
+            "fallback and every password derivation costs roughly 20x what it " +
+            "should — see the backend note in src/shared/vault.js",
+    );
+});
+
 test("wallet creation through the UI reaches the main view", async (env) => {
     await createWallet(env.page);
     const addrCount = await env.page