fix: declare toolbar icons and ship real PNGs in both archives (closes #371)

Neither manifest declared an `icons` block, so Chrome and Firefox both drew the
generic puzzle piece for this extension. That is the first thing the owner sees
on every launch, and an unbranded placeholder is also how a user fails to tell a
real extension from a look-alike.

Both manifests now declare 16/32/48/128 as `icons/icon<size>.png`, and the four
PNGs live at `icons/` in the tree. build.js copies them into each browser
directory relative to it, so nothing points up and out the way `dist/styles.css`
does, and the receipt records them like every other emitted file.

The sizes copied come from the manifest that will ship next to them, not from a
second list in build.js: a size a manifest declares and `icons/` does not hold
fails the build with the path that is missing, rather than emitting a directory
whose manifest references nothing. script/lib/package.js already resolved `.png`
strings, so an icon that reached a manifest but not the archive fails
self-containment; tests/packaging.test.js now pins that case, since it was
covered only incidentally before.

tests/manifest.test.js asserts the declaration in both manifests, that both
declare the same set, and that each referenced file is a PNG whose IHDR states
the size the entry claims — a declaration alone would still permit a reference
to a file that is not there or is not an image.

The artwork is original, drawn from geometry rather than traced or downloaded: a
flat dark-navy rounded square (#101A2E) with a teal (#35E0C2) triangular "A" —
one outer triangle minus a triangular counter — rasterised with 8x8 supersampling
and encoded as RGBA PNG. One shape, two flat colours, which is what a 16px
toolbar slot can carry.

Verified: make check 56 suites / 1023 tests, make build and make package green,
both archives unpacked and the four icons confirmed inside each with bytes
identical to the tree, make test-e2e 55/55 and 5/5, make test-e2e-firefox 8/8 and
7/7 (the latter installs the packaged XPI). Removing icons/icon48.png fails
make build; making build.js skip one copy fails make package with "the chrome
archive would not be self-contained: it is told to load icons/icon48.png, which
is not in it".

manifest/chrome.json

@@ -9,6 +9,12 @@
     "content_security_policy": {
         "extension_pages": "default-src 'self'; script-src 'self' 'wasm-unsafe-eval'; object-src 'self'; style-src 'self' 'unsafe-inline'; img-src 'self' data:; connect-src 'self' https: http:; frame-src 'none'; form-action 'none'; base-uri 'none'"
     },
+    "icons": {
+        "16": "icons/icon16.png",
+        "32": "icons/icon32.png",
+        "48": "icons/icon48.png",
+        "128": "icons/icon128.png"
+    },
     "action": {
         "default_popup": "src/popup/index.html"
     },

manifest/firefox.json

@@ -5,6 +5,12 @@
     "description": "Minimal Ethereum wallet for Firefox",
     "permissions": ["storage", "activeTab", "alarms", "<all_urls>"],
     "content_security_policy": "default-src 'self'; script-src 'self' 'wasm-unsafe-eval'; object-src 'self'; style-src 'self' 'unsafe-inline'; img-src 'self' data:; connect-src 'self' https: http:; frame-src 'none'; form-action 'none'; base-uri 'none'",
+    "icons": {
+        "16": "icons/icon16.png",
+        "32": "icons/icon32.png",
+        "48": "icons/icon48.png",
+        "128": "icons/icon128.png"
+    },
     "browser_action": {
         "default_popup": "src/popup/index.html"
     },