Add error.tsx and not-found.tsx to root and key route segments

Problem

Next.js App Router supports per-segment error.tsx and not-found.tsx files that gracefully render a UI when a server component throws or a route doesn't match. The repo currently has none — every uncaught server error renders the framework's default error screen, and any unknown route falls back to the framework's default 404. This is jarring for users and makes incident triage harder because there's no opportunity to log a correlation id or render a friendly retry CTA.

Evidence

• A repo-wide search for error.tsx, not-found.tsx returns no matches under src/app/.
• All segments rely on Next's defaults.

Proposed approach

1. Create src/app/error.tsx (root error boundary):

   • \"use client\" per Next.js docs.
   • Receives { error, reset } props.
   • Logs error.digest and error.message via the structured logger (or console.error for now).
   • Renders a shadcn/ui Card with title "Something went wrong", body containing error.digest (collapsed), and a button calling reset().

2. Create src/app/not-found.tsx:

   • Renders a shadcn/ui card with "Page not found" + a Link back to /.

3. Create segment-level error.tsx for each major route group, reusing a shared ErrorState component to keep them DRY:

   • src/app/assignments/error.tsx
   • src/app/budget/error.tsx
   • src/app/invoices/error.tsx
   • src/app/copilot/error.tsx
   • src/app/claude/error.tsx
   • src/app/reports/error.tsx
   • src/app/settings/error.tsx

4. Extract the shared component to src/components/error-state.tsx so per-segment files are 5–10 lines.

5. Make sure the error boundary preserves the sidebar layout — wrap ErrorState so it renders inside the existing <AppShell> rather than full-page.

6. Add a Playwright smoke test (1 spec, 2 cases) that:

   • Navigates to /__nope__ and asserts the not-found UI renders.
   • (Optional) hits a route segment with a deliberately throwing dev-only flag and asserts the error UI renders.

Acceptance criteria

• Root error.tsx and not-found.tsx exist and render the shared component.
• At least 5 segment-level error.tsx files cover the major route groups listed above.
• Shared ErrorState component is reused everywhere — no copy-pasted JSX.
• Visiting a non-existent path renders the new 404 UI inside the app layout.
• pnpm lint && pnpm typecheck pass.

Verification

1. pnpm dev, navigate to http://localhost:3000/__nope__ → expect the new 404 UI.
2. Temporarily throw new Error(\"boom\") at the top of any server component (e.g. src/app/copilot/page.tsx) → expect the segment error UI with a Try Again button. Revert.
3. Click "Try Again" → page re-renders without a hard reload.

Start with the listed src/app/error.tsx, src/app/not-found.tsx, route-segment files, and src/components/error-state.tsx, then inspect the existing AppShell and structured logger usage. Run pnpm lint && pnpm typecheck and the Playwright smoke test after verifying /nope renders the new 404 UI and the listed segments show the retry state inside the app layout.