Improved dashboard error msgs when credentials missing

[benvinegar]

Now when Counterscale is misconfigured, you get a far more human-readable error experience.

This PR also upgrades Vitest from 1.x to 2.x.

Before (v embarrassing):

After:

[codecov]

Codecov Report

Attention: Patch coverage is 41.66667% with 14 lines in your changes missing coverage. Please review.

Project coverage is 72.89%. Comparing base (0a19c38) to head (cf9fb9e).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
app/routes/dashboard.tsx	43.47%	13 Missing ⚠️
app/root.tsx	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #113      +/-   ##
==========================================
- Coverage   77.66%   72.89%   -4.77%     
==========================================
  Files          31       32       +1     
  Lines        3138     2225     -913     
  Branches      217      219       +2     
==========================================
- Hits         2437     1622     -815     
+ Misses        693      595      -98     
  Partials        8        8              

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