feat: add app router boundaries

[Joeloo1]

Description

This PR implements the missing App Router boundaries (not-found.tsx, error.tsx, global-error.tsx, and loading.tsx) to ensure a graceful and branded fallback experience across the application.

Changes Made

• app/not-found.tsx: Added a branded 404 page featuring the .landing-hero styling and a navigation path back to the main routes.
• app/error.tsx: Implemented a client-side error boundary that logs exceptions to console.error (preventing raw error exposure to users) and includes a reset() retry mechanism.
• app/global-error.tsx: Added a root layout fallback to catch critical failures, complete with its own HTML shell and a refresh option.
• app/loading.tsx: Created a simple layout-shift-free loading boundary where data fetching lands.

Acceptance Criteria Met

• All three boundaries implemented and styled to match the site.
• reset() retry works.
• No raw error text or stack traces shown in production.
• Errors reported to logging rather than swallowed.

Closes #50

[joelpeace48-cell]

@Joeloo1, please fix failing ci