refactor: standardize dev-playground on the app template

[MarioCadenas]

What

Standardizes apps/dev-playground on the databricks apps init template layout while keeping it a monorepo-linked dogfood app. No behavior/feature changes — same plugins, routes, and demos, restructured.

Why

The playground had drifted from the template (nested client/package.json, TanStack Router, bespoke tsconfig/deploy config-swaps). Aligning it means template regressions get dogfooded, and contributors see one canonical structure.

Changes

• Single package — fold the nested client/package.json in; adopt template tsconfig.{shared,server,client} + tsdown.server.config.ts + client vite.config; server entry → server/server.ts; pages at client/src/pages/<name>/<Name>Page.tsx.
• Router — TanStack Router → react-router 7 (createBrowserRouter in App.tsx, RootLayout/Outlet/NavLink, route errorElement); routeTree.gen.ts removed.
• Local SDK resolution via pnpm overrides — root pnpm.overrides pins @databricks/appkit{,-ui} → workspace:*; the app's package.json keeps the template's pinned version, so the app file is byte-identical to a scaffold but installs link into packages/.
• Client vite alias for appkit-ui — its exports development condition points at TS source that uses appkit-ui's internal @/ alias (unresolvable by an external bundler), so the client pins appkit-ui's built dist (kept fresh by build:watch; matches prod/deploy). Server/build resolve via the override alone.
• Deploy — deploy-playground.ts simplified for one package (both SDK tarballs into a single package.json; dropped the client/package.json injection + prepared-files/ config swaps). App build uses npm run for Databricks parity.
• Pre-existing drift fixes — surfaced by the newly tsc-gated build: appkit-ui chart prop renames (curveType→smooth, showDots→showSymbol, showLabel→showLabels), AgentEvent→AgentChatEvent, framework-provided plugin cache, ChartData/e.path guards.

Deviations from the template (monorepo-justified)

Stays pnpm (not npm), lints via root oxlint (no per-app eslint), keeps postcss tailwind, keeps vite dedupe for react, and omits the typegen lifecycle hooks (would break workspace install ordering).

Testing

• pnpm --filter=dev-playground typecheck — clean (cache-free)
• pnpm --filter=dev-playground build (tsc + tsdown + vite) — green
• oxlint + oxfmt --check — clean
• Verified on a live dev server: routes render, appkit-ui resolves to dist, no transform errors.
• Playwright E2E not run here (needs a workspace); unchanged.

Note for reviewers

pnpm -r typecheck shows a pre-existing failure in packages/appkit's own source (implicit-any + a shared/workspace-client resolution error) — independent of this PR (no packages/ files are touched; dev-playground passes within that run).

Known follow-up (not in this PR)

appkit.files is inferred by createApp as a non-callable union (FilesExport | agents registry) with this plugin set; worked around with a single Extract<> narrowing in server.ts. Looks like an appkit-side inference issue worth a real fix.

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📦 Try this PR's app template

Scaffolds a new app from this PR's SDK build. Run it in any folder (requires the GitHub CLI — gh auth login — and the Databricks CLI):

gh run download 32252985324 -R databricks/appkit -n appkit-template-0.61.1-pr.94abf32-playground-standard-541 -D appkit-pr-541 \
  && unzip -o "appkit-pr-541/appkit-template-0.61.1-pr.94abf32-playground-standard-541.zip" -d "appkit-pr-541" \
  && databricks apps init --template "appkit-pr-541"

The template pins @databricks/appkit and @databricks/appkit-ui to tarballs built from this branch, so the scaffolded app runs against this PR's code.