build: run the browser e2e suites in CI (closes #259)

Nothing automatic ran either e2e suite, so every browser-level guarantee in this
wallet -- WebAssembly under the shipped CSP, the recovery-phrase and private-key
DOM wipes, the ConfirmTx spend gate, the dApp approval round trips -- held only
when a human or an agent remembered to run it by hand.

.gitea/workflows/e2e.yml adds two jobs, e2e-chrome and e2e-firefox, one per
browser so a Chrome failure cannot hide the Firefox result. They are separate
from the check workflow: REPO_POLICIES.md caps make test at 20 seconds and
script/cibuild is a docker build whose Dockerfile runs make check, so neither the
cap nor the local fast path is touched. make check is byte-for-byte unchanged.

Neither suite could run on the runner as it stood, and the reason is not
docker-in-docker. The runner executes a job inside a container against the HOST's
docker daemon, and the job's checkout lives on a docker volume rather than a host
path, so `docker run -v "$PWD:/work"` is resolved by the host, silently succeeds
and mounts an empty directory -- measured on this runner. The runner image's node
is also too old to install this repo's dependencies. Both suites therefore ship
the repo to the daemon as a build context and build the extension inside the
pinned image, which leaves docker as the only prerequisite on a runner or a
laptop. The suites themselves are unchanged; only how the repo reaches the
container is.

Both scripts now build with --iidfile and run the image by ID rather than by tag,
so two clones running a suite at once on the same host cannot swap it under each
other.

The jobs report, they do not gate. Whether a check blocks a merge is Gitea branch
protection, which this repo does not configure, so a failure is a red mark a
reviewer must account for. Nothing can pass vacuously: no continue-on-error, no
`|| true`, and both scripts exit non-zero when docker is missing, when the image
build fails and when the browser fails to start.

tests/e2e/firefox/Dockerfile

@@ -1,10 +1,24 @@
 # Firefox end-to-end image: stock Firefox plus geckodriver on a node base,
-# built by script/test-e2e-firefox. The repo is bind-mounted at /work; the
-# harness itself has no dependencies, so nothing is installed for it.
+# with this repo and a freshly built extension inside it, built by
+# script/test-e2e-firefox. The harness itself has no dependencies, so
+# nothing is installed for it.
 #
-# All three external artifacts are pinned by digest. The Firefox version in
-# particular must not float: -remote-allow-system-access is mandatory on 153
-# and was not on 142, so the flag the harness passes is version-coupled.
+# The build context is the repo root. The repo is baked in rather than
+# bind-mounted because a bind mount does not resolve under Gitea Actions:
+# the runner runs the job in a container against the HOST's docker socket,
+# so the source side of a -v is resolved by the host daemon while the job's
+# checkout lives on a docker volume that is not a host path -- the mount
+# silently succeeds and /work is empty. Baking the build in is also the
+# only way this suite can have both a built extension and the
+# `--network none` it runs under, since a container with no network cannot
+# install anything.
+#
+# All three external artifacts are pinned by digest, and are fetched in
+# layers above the repo copy, so editing the harness or any source file
+# re-runs only the two cheap layers at the bottom. The Firefox version in
+# particular must not float: -remote-allow-system-access is mandatory on
+# 153 and was not on 142, so the flag the harness passes is
+# version-coupled.
 
 # node:22-bookworm-slim, 2026-08-12
 FROM node@sha256:d649c27dae7ba0137b3cef5dd75baa422c08dc3d9e3fc0c23dfb172dc3cc6436
@@ -48,4 +62,16 @@ ENV FIREFOX_BIN=/opt/firefox/firefox
 ENV GECKODRIVER=/usr/local/bin/geckodriver
 
 WORKDIR /work
+
+# Same layering as the root Dockerfile: script/bootstrap installs the
+# prerequisites and the dependencies, and the manifests are copied first so
+# that layer is cached until they change.
+COPY script/ script/
+COPY package.json yarn.lock ./
+RUN script/bootstrap
+
+COPY . .
+
+RUN make build
+
 CMD ["node", "tests/e2e/firefox/run.js", "dist/firefox"]