[Feature]: Add a proper 404 / Not Found page

### Problem Statement

The app currently renders a bare, unstyled placeholder for unmatched routes. In src/routes/__root.tsx, the root route's notFoundComponent is:

notFoundComponent: () =>

Navigating to any unknown route (e.g. /some-missing-page) shows plain "Not Found" text with no styling and no way back into the app. It looks broken and is inconsistent with the rest of the UI.

### Proposed Solution

Replace the placeholder with a styled 404 page that:

- Uses the existing DaisyUI + Tailwind design tokens (bg-base-200, text-base-content, btn btn-primary, etc.) so it respects the current light/dark theme
- Provides a clear "back to app" action linking to /trackpad
- Routes its text through the existing i18n helper (t(...)) instead of hardcoding strings

The errorComponent right next to it is a similar bare stub (Error: {message}); happy to polish that in the same PR if maintainers want, or keep this scoped to the 404 only.

### Alternatives Considered (Optional)

Leaving the default placeholder as-is — rejected since it reads as a broken page and doesn't match the app's design.

### Additional Context (Optional)

Picked this up from a team meeting where the maintainers asked me to take it on. Opening this to track it; PR to follow.

### Checklist

- [x] I have searched for existing feature requests to avoid duplicates