fix(workflow-filter): field should appear on first load

[cheefbird]

Context

workflow-filter wasn't showing up on first navigation to the actions page in firefox. required a manual refresh to appear.

root cause: findNavList() ran a synchronous querySelector that returned null because WXT's location change event fires before github's react finishes rendering the sidebar.

note: chrome was unaffected because it uses the Navigation API (event-driven) while firefox falls back to polling, hitting a different timing window.

Closes: #49

Changes Made

src/lib/dom.ts — new waitForElement<T>() utility. MutationObserver-based, resolves when selector appears in DOM. integrates with AbortSignal for cleanup, returns null on abort. generic type param for typed returns.

src/lib/dom.test.ts — 8 tests covering fast-path, async detection, timeout, abort, pre-aborted signal, nested elements, observer cleanup verification, and element+abort race condition. uses happy-dom environment.

src/entrypoints/grody.content/features/workflow-filter/index.ts — init is now async, uses waitForElement instead of synchronous findNavList(). dev-only console.warn when nav element isn't found (tree-shaken in prod). showMore/totalPages checks remain synchronous since they're children of the nav element that was just resolved.

package.json — added happy-dom dev dep for DOM test environment.

Testing Notes

• load extension in firefox, navigate to a repo's actions page via link (not direct URL) — filter bar should appear without refresh
• verify chrome still works as before (no regression)
• verify filter bar still doesn't render on repos with <= 1 page of workflows
• pnpm test — 92 tests passing
• pnpm lint — clean
• pnpm check — clean