feat(web): add dialectic reasoning playground

Lets users fire one query at every Honcho reasoning level (minimal / low /
medium / high / max) in parallel and compare answers + latency side-by-side
in a 5-column grid.

- New route: /workspaces/:ws/peers/:peer/playground
- New component: DialecticPlayground with per-column state, level checkboxes
  to skip expensive levels, and a single Run button that fans out via
  Promise.all
- Extends useChat() to accept an optional reasoning_level parameter (was
  hardcoded to "low")
- Adds a Playground button next to Chat on the peer detail page
- Tests cover the parallel fan-out (peak concurrency = 5, no serial
  awaiting) and per-column latency measurement
- Screenshot Playwright spec (idle / mid-flight / settled) drives the
  images in docs/screenshots/ — regen with PLAYWRIGHT_BASE_URL=... pnpm
  exec playwright test e2e/playground.screenshots.spec.ts

packages/web/src/routes/workspaces_.$workspaceId_.peers_.$peerId_.playground.tsx

@@ -0,0 +1,6 @@
+import { createFileRoute } from "@tanstack/react-router";
+import { DialecticPlayground } from "@/components/playground/DialecticPlayground";
+
+export const Route = createFileRoute("/workspaces_/$workspaceId_/peers_/$peerId_/playground")({
+	component: DialecticPlayground,
+});