fix(api): allow ui refresh to deep links

[zkhvan]

After #1833, installing the chart into a cluster will cause links to nested paths to return 404.

Reproduction

• Install 0.6.0-unstable-20240419
• Navigate to Kargo Dashboard
• Create new project (kargo-demo)
• Navigate to new project (https://host/project/kargo-demo)
• Refresh and you'll see:
  404 page not found

Since assets are served through the API server, the router needs to redirect the URLs to index.html since the UI is a single page application.

@krancour I figured you'd have context on this, so tagging you. I'm not sure if this is something you've encountered and were thinking of an alternative solution.

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[krancour]

Nice find. I'll review shortly!

[krancour]

A note about how this escaped my attention...

When we run Kargo from source with tilt up, the UI isn't served from the API server like it normally would be, because front-end development is faster if the UI is served from its own container running vite, which can hot-reload UI changes...

The embedded-ness of the UI was tested by running the kargo server command. I could not progress past the login page because the local server starts with no authn methods enabled and the UI doesn't support that yet. (#1728 is still WIP.)

So... being unable to progress past the login page, I never actually did see anything deeper than the login page served from the embedded FS. Especially since assets loaded, it didn't occur to me that anything was amiss.

Installing the unstable chart does indeed permit me to easily reproduce the problem.

[krancour]

@zkhvan I proposed minor tweaks for readability and if they're cool with you then this PR lgtm.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 39 lines in your changes are missing coverage. Please review.

Project coverage is 46.26%. Comparing base (fe958c8) to head (eebf8a5).
Report is 2 commits behind head on main.

Files	Patch %	Lines
internal/api/server.go	0.00%	39 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1878      +/-   ##
==========================================
- Coverage   46.38%   46.26%   -0.12%     
==========================================
  Files         215      215              
  Lines       14066    14101      +35     
==========================================
  Hits         6524     6524              
- Misses       7243     7278      +35     
  Partials      299      299              

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