[BDGR-179] Remove YSTV login

[markspolakovs]

It's janky and unused by anyone (including YSTV).

Note that this keeps around username/password login, as it's still used in E2E tests, however it doesn't implement an username/password authentication backend - the only supported ones are Google and Dummy Test Auth. If there's demand we might consider adding a local user database.

Fixes BDGR-179.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 22.28%. Comparing base (79b5353) to head (1427cb8).
Report is 66 commits behind head on main.

Files	Patch %	Lines
server/app/login/@local/form.tsx	0.00%	2 Missing ⚠️
server/app/login/@local/page.tsx	0.00%	1 Missing ⚠️
server/app/login/layout.tsx	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            main     #484       +/-   ##
==========================================
+ Coverage   3.06%   22.28%   +19.21%     
==========================================
  Files         88       95        +7     
  Lines       2548     3595     +1047     
  Branches     546      604       +58     
==========================================
+ Hits          78      801      +723     
- Misses      2470     2647      +177     
- Partials       0      147      +147     

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