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/DiffView.tsx

@@ -1,10 +1,10 @@
 // A `Diff` content block, rendered as coloured hunks. ACP hands over whole
 // `oldText`/`newText`, so the lines and their `+added −removed` counts are
-// computed here (structuredDiff.ts) rather than parsed out of a patch.
+// computed here (diff.ts) rather than parsed out of a patch.
 
 import { useMemo } from "react";
 
-import { computeDiffLines, diffStats, type DiffLine } from "../../structuredDiff.js";
+import { computeDiffLines, diffStats, type DiffLine } from "./diff.js";
 import type { AcpDiff } from "../../types.js";
 import { Copyable } from "./Copyable.js";