feat(ext/shared): centralize error-message copy in ERROR_COPY map

Replaces the popup's regex-chain humanizeError with a total lookup over
every error code returned by extension/src/service-worker/router/. A
generated test discovers codes via grep so the registry can't drift.
The popup keeps its small set of regex translators for Rust/serde error
phrasing that doesn't go through the router's error vocabulary.

Subsumes B2 — fullscreen consumer lands in the next commit.

extension/src/shared/__tests__/error-copy.test.ts

@@ -0,0 +1,44 @@
+import { describe, it, expect } from 'vitest';
+import { execSync } from 'node:child_process';
+import { resolve } from 'node:path';
+import { ERROR_COPY, lookupErrorCopy } from '../error-copy';
+
+const repoRoot = resolve(__dirname, '../../../..');
+
+function discoverCodes(): Set<string> {
+  const out = execSync(
+    `grep -rohE "ok: false, error: '[^']+'" extension/src/service-worker/ \
+       --include="*.ts" --exclude-dir=__tests__`,
+    { cwd: repoRoot, encoding: 'utf-8' },
+  );
+  const codes = new Set<string>();
+  for (const line of out.split('\n')) {
+    const m = line.match(/error: '([^']+)'/);
+    if (m) codes.add(m[1]);
+  }
+  return codes;
+}
+
+describe('ERROR_COPY', () => {
+  it('contains an entry for every error code returned by the service worker', () => {
+    const discovered = discoverCodes();
+    expect(discovered.size).toBeGreaterThan(0);
+    const missing: string[] = [];
+    for (const code of discovered) {
+      if (!ERROR_COPY[code]) missing.push(code);
+    }
+    expect(missing).toEqual([]);
+  });
+
+  it('lookupErrorCopy returns the mapped entry for known codes', () => {
+    const copy = lookupErrorCopy('vault_locked');
+    expect(copy.title).toBe('Vault locked');
+    expect(copy.body).toMatch(/unlock/i);
+  });
+
+  it('lookupErrorCopy falls back to a generic shape for unknown codes', () => {
+    const copy = lookupErrorCopy('made_up_code_xyz');
+    expect(copy.title).toBeTruthy();
+    expect(copy.body).toContain('made_up_code_xyz');
+  });
+});