build: add ESLint to script/lint and containerize linting (closes #152)

script/lint ran `prettier --check .`, byte for byte what script/fmt-check
runs, so make check checked formatting twice and did no static analysis on
a cryptocurrency wallet. Two used-but-not-imported crashes shipped past it.

ESLint is pinned in package.json with @eslint/js recommended as the base and
a flat config in eslint.config.js. no-undef and no-unused-vars are restated
error-level so a future recommended-set change cannot downgrade them.
Globals are declared per tree rather than globally, because a too-wide set
hides the next unimported identifier: browser for the popup and content
scripts, service worker for src/background/ and src/shared/, browser for the
one documented POPUP ONLY module in src/shared/, jest for tests/, node for
build.js, and both for the e2e harnesses, which carry the callbacks they
ship into the page inline.

Two rules new to the recommended set are off, and both would have cost
something to satisfy. no-useless-assignment flags the `password = null` and
`decryptedSecret = null` wipes in approval.js and confirmTx.js: those
assignments are dead by construction, which is the point of them, and the
rule's fix is to delete the wipe. preserve-caught-error would change what
the wallet's error paths throw, which is a decision of its own.

Every remaining violation is fixed: 41 unused bindings and 53 undefined
identifiers. Unused catch bindings became `catch {`, which the repo already
used; the shared init(ctx) view signature keeps its parameter as _ctx in the
three views that do not read it. src/shared/uniswap.js keeps its unused
V2_SWAP_EXACT_OUT decoder behind a scoped disable, because deleting it would
widen the gap it represents rather than close it (#283).

Linting is containerized. script/lint builds the Dockerfile's new lint stage
so the ESLint deciding whether this repo is green is the pinned one and not
whatever the host has; AUTISTMASK_LINT_NATIVE, set only in that image, is
what makes make check inside the CI build lint in place instead of recursing
into docker. The check stage takes a COPY --from=lint dependency so a lint
failure fails the whole build early rather than racing it.

No --fix anywhere in the lint path: make check remains non-mutating.

The README claim that a used-but-not-imported identifier is invisible to
make check, and the same claim in script/test-e2e, are no longer true and
are corrected.

eslint.config.js

@@ -0,0 +1,144 @@
+// ESLint flat config. Static analysis for make check; formatting stays with
+// prettier (script/fmt-check), so nothing here touches style.
+//
+// The sources are CommonJS and are bundled per entrypoint by build.js, so the
+// globals differ by tree and are declared per tree below. Getting that wrong in
+// either direction defeats the point: too few globals buries a real no-undef in
+// false positives, too many hides the next unimported identifier.
+
+const js = require("@eslint/js");
+const globals = require("globals");
+
+// The extension APIs. MV3 Chrome exposes `chrome`; Firefox exposes both, and
+// the code feature-detects between them.
+const extensionGlobals = {
+    chrome: "readonly",
+    browser: "readonly",
+};
+
+const commonjs = {
+    ecmaVersion: 2024,
+    sourceType: "commonjs",
+};
+
+module.exports = [
+    {
+        ignores: [
+            "dist/",
+            "node_modules/",
+            // Emitted by build.js, not authored here.
+            "src/popup/styles/",
+        ],
+    },
+
+    js.configs.recommended,
+
+    {
+        rules: {
+            // The two rules this config exists for. Both are already
+            // error-level in the recommended set; restated so a future
+            // recommended-set change cannot silently downgrade them.
+            "no-undef": "error",
+            // `_`-prefixed arguments are the deliberate "present for the
+            // interface, unused here" marker: the popup views share one
+            // init(ctx) signature and three of the eight do not read ctx.
+            // An unused catch binding is written `catch {`, which the repo
+            // already does, so caught errors stay checked.
+            "no-unused-vars": ["error", { argsIgnorePattern: "^_" }],
+
+            // Off: it flags `password = null` and `decryptedSecret = null` in
+            // approval.js and confirmTx.js, which are the best-effort wipes of
+            // decrypted key material after use. The assignments are dead by
+            // construction — that is the point of them — and satisfying the
+            // rule would mean deleting the wipes.
+            "no-useless-assignment": "off",
+
+            // Off: it requires every rethrow to carry `{ cause }`. That is a
+            // change to what the wallet's error paths actually throw, which is
+            // not this config's business; adopting it is its own decision.
+            "preserve-caught-error": "off",
+        },
+    },
+
+    // Popup and content scripts: page/window context.
+    {
+        files: ["src/popup/**/*.js", "src/content/**/*.js"],
+        languageOptions: {
+            ...commonjs,
+            globals: { ...globals.browser, ...extensionGlobals },
+        },
+    },
+
+    // MV3 background: a service worker, with no window and no document.
+    {
+        files: ["src/background/**/*.js"],
+        languageOptions: {
+            ...commonjs,
+            globals: { ...globals.serviceworker, ...extensionGlobals },
+        },
+    },
+
+    // src/shared is bundled into both, so it may only use what both provide:
+    // the service worker globals are the intersection, plus the extension APIs.
+    {
+        files: ["src/shared/**/*.js"],
+        languageOptions: {
+            ...commonjs,
+            globals: { ...globals.serviceworker, ...extensionGlobals },
+        },
+    },
+
+    // src/shared/ens.js is the documented exception to the line above: its own
+    // header says POPUP ONLY, it caches in localStorage, and only popup views
+    // require it. Linting it as a service worker would be wrong about the file.
+    {
+        files: ["src/shared/ens.js"],
+        languageOptions: {
+            ...commonjs,
+            globals: { ...globals.browser, ...extensionGlobals },
+        },
+    },
+
+    // Unit tests: jest on node.
+    {
+        files: ["tests/**/*.test.js"],
+        languageOptions: {
+            ...commonjs,
+            globals: { ...globals.node, ...globals.jest },
+        },
+    },
+
+    // The build script is a plain node program.
+    {
+        files: ["build.js"],
+        languageOptions: {
+            ...commonjs,
+            globals: { ...globals.node },
+        },
+    },
+
+    // The e2e harnesses are node programs that also carry, inline, the
+    // callbacks they ship into the browser via page.evaluate — so both
+    // contexts really are present in the same file and both sets of globals
+    // are in scope somewhere in it.
+    {
+        files: ["tests/e2e/**/*.js"],
+        languageOptions: {
+            ...commonjs,
+            globals: {
+                ...globals.node,
+                ...globals.browser,
+                ...extensionGlobals,
+            },
+        },
+    },
+
+    // This config file itself.
+    {
+        files: ["eslint.config.js"],
+        languageOptions: {
+            ...commonjs,
+            globals: { ...globals.node },
+        },
+    },
+];