fix(frontend): add 404 fallback route for unknown paths

[shiva9198]

Description

Fix invalid frontend routes showing a blank screen by adding a wildcard route and a dedicated 404 page with a safe navigation path back to home.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)

Related Issues

Fixes #6336

Changes Made

• Added wildcard route (path="*") in core/frontend/src/App.tsx
• Added new core/frontend/src/pages/not-found.tsx
• Rendered user-friendly 404 UI with “Back to Home” link

Testing

Describe the tests you ran to verify your changes:

• Unit tests pass (cd core && pytest tests/)
• Lint passes (cd core && ruff check .)
• Manual testing performed

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Screenshots (if applicable)

• Added screenshot(s) showing invalid route now renders 404 page instead of blank screen.

[github-actions]

PR Requirements Warning

This PR does not meet the contribution requirements.
If the issue is not fixed within ~24 hours, it may be automatically closed.

Missing: No linked issue found.

To fix:

1. Create or find an existing issue for this work
2. Assign yourself to the issue
3. Re-open this PR and add Fixes #123 in the description

Exception: To bypass this requirement, you can:

• Add the micro-fix label or include micro-fix in your PR title for trivial fixes
• Add the documentation label or include doc/docs in your PR title for documentation changes

Micro-fix requirements (must meet ALL):

Qualifies	Disqualifies
< 20 lines changed	Any functional bug fix
Typos & Documentation & Linting	Refactoring for "clean code"
No logic/API/DB changes	New features (even tiny ones)

Why is this required? See #472 for details.

[Copilot]

Pull request overview

Adds a client-side 404 fallback so unknown frontend routes render a dedicated “Not Found” page instead of leaving users on a blank / unhandled route.

Changes:

• Added a new NotFound page component.
• Registered a catch-all (*) route in App.tsx to render the 404 page for unknown paths.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
core/frontend/src/pages/not-found.tsx	Introduces the 404 UI and a link back to the home route.
core/frontend/src/App.tsx	Adds the wildcard route and imports the new NotFound page.

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[shiva9198]

@copilot open a new pull request to apply changes based on the comments in this thread