test(mobile): serve the xterm vendor bundles the browser suite needs

The mobile suite drives a real browser against a WebServer started from
TypeScript source, so fastify-static serves join(__dirname, 'public') =
src/web/public — not dist/web/public, where `npm run build` puts the vendor
bundles. Every /vendor/xterm* request 404s, so `Terminal` is never defined,
initTerminal() never runs, and any test touching app.terminal dies with
"Cannot read properties of null".

Measured in one worktree, toggling only the vendor files:

  before: 404s=5  Terminal=undefined  app.terminal=null   8 failed | 26 passed
  after:  404s=0  Terminal=function   app.terminal=live   6 failed | 28 passed

The 6 remaining failures are genuine pre-existing bugs (stale layout and
accessory-bar expectations, a CJK timeout) and are left alone here.

This went unnoticed because config/vitest.ci.config.ts excludes test/mobile/**,
so CI never ran the suite. `npm run test:mobile` now runs it, with a pretest
hook that builds the bundles.

The asset list was derived from the actual 404s rather than from build.mjs —
which is how xterm-addon-unicode11 and xterm-zerolag-input got included; reading
the build file alone would have missed both. Outputs go to the gitignored
src/web/public/vendor/, so they stay build artifacts. The script is idempotent
(skips outputs newer than their source) and does not touch the normal build.

Full CI suite unchanged: 4368 passed.

package.json

@@ -21,6 +21,8 @@
     "test:watch": "vitest --config config/vitest.config.ts",
     "test:coverage": "vitest run --config config/vitest.config.ts --coverage",
     "test:ci": "vitest run --config config/vitest.ci.config.ts",
+    "pretest:mobile": "node scripts/prepare-test-vendor.mjs",
+    "test:mobile": "vitest run --config test/mobile/vitest.config.ts",
     "check:frontend-syntax": "node scripts/check-frontend-syntax.mjs",
     "fix:node-pty": "node scripts/fix-node-pty.mjs",
     "typecheck": "tsc --noEmit",