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/dialogs/HelpOverlay.tsx

@@ -6,7 +6,7 @@ import { useEffect, useState } from "react";
 import { createPortal } from "react-dom";
 
 import { CommandSet, defineCommands, keyCaps } from "../../commands.js";
-import { groupedCommands } from "../../help.js";
+import { groupedCommands } from "./model.js";
 
 export function HelpOverlay() {
   const [open, setOpen] = useState(false);