feat(maestro): wire React app as sole web entry point

Deletes the vanilla shell and mounts App.tsx on #root. main.ts and the
23 views it drove are gone; App.tsx composes the ported components —
sidebar, multi-view, command bar, spawn modal, help and restart
overlays, daemon link, drawer — around the catalog and layout stores.

The pure parts of four deleted modules moved next to the components
that still use them: the spawn-form model and spawnSimilar into
components/spawn, the transcript's markdown/format helpers and the diff
into components/structured, the help overlay's grouping into
components/dialogs. Document-level event names now live in uiEvents.ts.

Deduplicated two pairs the migration left side by side: the sidebar
uses dialogs/KillDialog and common/ContextMenu. It also listens for the
command bar's kill/rename events, which nothing had picked up since
catalog.ts went away.

styles.css stays as the token and base layer, with `#app` renamed to
`#root` and the pane-body rules re-keyed off the classes the React
panes actually carry.

Closes #253

web/src/components/structured/model.ts

@@ -1,7 +1,7 @@
 // Pure transcript logic for the structured pane (#246) — the decisions
 // docs/STRUCTURED-VIEW.md leaves to a client, kept out of the components so
 // they can be tested without a DOM or a socket. Ported from structured.ts's
-// private helpers and the parts of structuredRender.ts that were data, not
+// private helpers and the parts of the old structuredRender.ts that were data, not
 // markup.
 
 import type {
@@ -14,7 +14,7 @@ import type {
   TranscriptRowKind,
   TurnMeta,
 } from "../../types.js";
-import { computeDiffLines, diffStats } from "../../structuredDiff.js";
+import { computeDiffLines, diffStats } from "./diff.js";
 
 /** Human label behind a `ToolKind`. An unrecognised/absent kind is "Tool" —
  *  never a blank label. */