Fix/service worker offline fallback

[DhyaniKavya]

issue : #5874

Problem:-
The service worker references an offlineFallbackPage variable when handling failed network requests:
=> const fallbackResponse = await caches.match(offlineFallbackPage);

However, offlineFallbackPage is only declared in a lint hint comment:
=> /* global offlineFallbackPage */

and is never defined at runtime. This can lead to a ReferenceError when the service worker attempts to serve a fallback response during offline conditions.
-Offline navigation may fail silently
-Users may see a blank page when offline
-PWA reliability is degraded

Solution:-
-This PR defines the fallback page variable to ensure the service worker can safely return a cached response when offline.
-defines offlineFallbackPage in sw.js
-uses an existing cached resource (./index.html) as fallback
-preserves existing fetch handler logic
-prevents runtime errors during offline fallback

Changes Made:-
-File modified:
->sw.js

• defined offlineFallbackPage constant
• ensured fallback resource matches precached file

ESLint: ✅ no lint errors
Tests: ⚠️ pre-existing failures on master (unrelated to this change)

Impact:-
-Prevents runtime errors in offline mode
-Improves Progressive Web App reliability
-Ensures graceful fallback when offline
-No functional or behavioral regressions

Notes:-
This change is intentionally minimal and non-breaking. It defines the expected fallback variable while preserving existing service worker logic and caching behavior.

Happy to refine further if maintainers prefer a dedicated offline page in the future.

[github-actions]

❌ Some Jest tests failed. Please check the logs and fix the issues before merging.

Failed Tests:

palette.test.js
logo.test.js
logoconstants.test.js

[github-actions]

❌ Some Jest tests failed. Please check the logs and fix the issues before merging.

Failed Tests:

palette.test.js
logo.test.js
logoconstants.test.js

[vyagh]

pre-existing jest failures??

[Chaitu7032]

I think they are mostly reference and type errors .

[DhyaniKavya]

@omsuneri The failing Jest tests appear to be pre-existing and reproducible on the current master branch.
This PR only defines the offlineFallbackPage constant in sw.js and does not touch logic covered by these tests.

[github-actions]

✅ All Jest tests passed! This PR is ready to merge.

[DhyaniKavya]

@omsuneri @walterbender Rebased the branch successfully and all tests are passing now . Please review the changes, if anything needs change, i am open to address it. Thankyou.