test(e2e): add axe-core accessibility fixture and smoke spec

Adds @axe-core/playwright with a shared fixture providing an `axe`
helper. New a11y.spec.ts runs WCAG 2.1 AA checks on signin, dashboard,
timeline, allocations, resources, and projects pages. Currently reports
violations as warnings — upgrade to hard failures after fixes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/web/e2e/a11y-fixture.ts

@@ -0,0 +1,24 @@
+import { test as base, expect } from "@playwright/test";
+import AxeBuilder from "@axe-core/playwright";
+
+/**
+ * Shared Playwright fixture that adds an `axe` helper to every test.
+ * Usage:
+ *   import { test, expect } from "./a11y-fixture.js";
+ *   test("page is accessible", async ({ axe }) => {
+ *     const results = await axe.analyze();
+ *     expect(results.violations).toEqual([]);
+ *   });
+ */
+export const test = base.extend<{ axe: AxeBuilder }>({
+  axe: async ({ page }, use) => {
+    const builder = new AxeBuilder({ page })
+      // Exclude known third-party widgets that we cannot control
+      .exclude("#__next-build-indicator")
+      // Only check WCAG 2.1 AA — the standard most teams target
+      .withTags(["wcag2a", "wcag21a", "wcag2aa", "wcag21aa"]);
+    await use(builder);
+  },
+});
+
+export { expect };