Add declarative DOM helpers for UI code

- New src/client/dom.ts with el() hyperscript builder, show/hide/visible
  display toggles, and byId() typed element lookup
- Add jsdom dev dependency for DOM testing
- 29 tests covering element creation, event wiring, visibility, and lookups
- Update CODING_STANDARDS.md with DOM helper guidance
- Apply Biome auto-format to client and server files

Author: rgilks

docs/CODING_STANDARDS.md

@@ -35,6 +35,16 @@ Classes are acceptable in places like:
 
 These files coordinate long-lived state, timers, DOM, canvas, sockets, or platform APIs. That is a legitimate use of classes in this project.
 
+### DOM helpers
+
+Use `src/client/dom.ts` helpers for declarative DOM construction in UI code:
+
+- **`el(tag, props, ...children)`** — Create elements with class, text, handlers, and children in one expression instead of multi-line createElement/className/addEventListener/appendChild chains.
+- **`visible(el, condition)` / `show(el)` / `hide(el)`** — Toggle display instead of writing `.style.display = condition ? 'block' : 'none'` everywhere.
+- **`byId(id)`** — Typed `getElementById` that throws on missing elements, replacing `document.getElementById('x')!` non-null assertions.
+
+Prefer `el()` for building element trees programmatically. Continue using `innerHTML` for complex HTML templates where `el()` would be awkward.
+
 ## Refactoring Guidance
 
 - Prefer extracting pure helper modules before introducing new patterns or libraries.