Add a global App Router error boundary and not-found page

[mikewheeleer]

Implement global error and not-found boundaries

Description

The app has route-level loading.js files under app/invest/loading.js and app/invoices/loading.js but no error.js or not-found.js, so an unexpected render error or unknown URL falls back to Next.js defaults that clash with the dark slate/cyan theme. This issue adds branded error and 404 boundaries that reuse ErrorBanner.

Requirements and context

• Repository scope: Liquifact/Liquifact-frontend only.
• Create app/error.js (client component) using components/ErrorBanner.jsx with a reset action and the dark theme background.
• Create app/not-found.js with a link back home and consistent styling.
• Optionally add app/global-error.js for layout-level failures.
• Reuse copy/en.js strings; add any new copy keys there rather than inlining text.

Suggested execution

• Fork the repo and create a branch
• git checkout -b feature/routing-09-error-notfound-boundaries
• Implement changes
  • Write code in: create app/error.js, app/not-found.js; update app/copy/en.js.
  • Write comprehensive tests in: create app/error.test.tsx and app/not-found.test.tsx.
  • Add documentation: document the boundaries in README.md.
  • Add JSDoc on the reset handler.
  • Validate a11y: error region uses role="alert" (already in ErrorBanner) and 404 has a focusable home link.
• Test and commit

Test and commit

• Run npm run lint, npm test, and npm run build.
• Cover edge cases: reset re-render and unknown route navigation.
• Include the full npm test output.

Example commit message

feat: add global error boundary and branded not-found page

Guidelines

• Minimum 95 percent test coverage for impacted modules.
• Clear, reviewer-focused documentation.
• Timeframe: 96 hours.

Community & contribution rewards

• 💬 Join the Liquifact community on Discord for questions, reviews, and faster merges: https://discord.gg/JrGPH4V3
• ⭐ This is a GrantFox OSS / Official Campaign task and may be rewarded. When your PR is merged you'll be prompted to rate the project — if this issue and the maintainers helped you ship, we'd be grateful for a 5-star rating. Clear questions in Discord and tidy, well-tested PRs are the fastest path to a merge and a reward.

[theFirstCodeManiac]

@theFirstCodeManiac has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Boss, please assign this to me.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @theFirstCodeManiac to this issue.