Redesign web inspector UI: 3-panel layout, templates, external CSS

- index.html: minimal shell with <template> elements for all repeated
  DOM structures (node-item, device-group, device-item, ctrl-group,
  ctrl-row, capture-badge); links external style.css
- style.css: all styles extracted from index.html
- lib/dom.mjs: by_id, qs, clone, show, hide helpers
- app.mjs: persistent SSE node list replaces Discover button; clicking
  a node connects to it; uses clone()/replaceChildren() throughout;
  no innerHTML for structure; event wiring at bottom

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

dev/web/public/lib/dom.mjs

@@ -0,0 +1,5 @@
+export const by_id = id  => document.getElementById(id);
+export const qs    = (sel, root = document) => root.querySelector(sel);
+export const clone = id  => document.getElementById(id).content.cloneNode(true).firstElementChild;
+export const show  = el  => el.removeAttribute('hidden');
+export const hide  = el  => el.setAttribute('hidden', '');