feat(bff): proper support for SPA routes

[ederign]

Description

This PR sits on top #699 and does proper handling of SPA Routes.

In this PR: (specific commit a7448dd)

• app.go: add a fallback if a path does not exist to app.config.StaticAssetsDir/index.html
• app_test.go: test the fallback (we should not get a 404 like before).

How Has This Been Tested?

In the video, every time the UI blinks, I reload it. Also, pay attention that on /api/v1/something I still keep using our default api 404 page.

Kapture.2025-01-14.at.17.06.19.mp4

Merge criteria:

• All the commits have been signed-off (To pass the DCO check)

Please merge #699 first!!!!

• The commits have meaningful messages; the author will squash them after approval or in case of manual merges will ask to merge with squash.
• Testing instructions have been added in the PR body (for PRs involving changes that are not immediately obvious).
• The developer has manually tested the changes and verified that the changes work.
• Code changes follow the kubeflow contribution guidelines.

[ederign]

@Griffin-Sullivan I just fixed the build.

[ederign]

@Griffin-Sullivan rebased!

[Griffin-Sullivan]

Works for me! Tested in Kubeflow.

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Griffin-Sullivan

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[Griffin-Sullivan]

/lgtm