tests: Restore web unit coverage in CI

[nishantmonu51]

Restore deterministic frontend unit execution and publish separate LCOV signals.

• keep web-admin Vitest discovery separate from Playwright suites
• add and pin V8 coverage support for web-admin and web-common
• run both unit suites in the foreground under the repository-pinned Node 22
• upload separate web-admin and web-common Codecov flags
• compare bookmark URL parameters semantically and complete legacy bookmark defaults
• correct incremental-model fixtures uncovered by the restored suite

Validation:

• mise x node@22 -- npm run test:unit -w web-admin — 333 tests passed
• mise x node@22 -- npm run test -w web-common — 2,958 tests passed
• mise x node@22 -- npm run coverage -w web-admin
• mise x node@22 -- npm run coverage -w web-common

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. No documentation update is required.
• Intend to cherry-pick into the release branch
• I am proud of this work!

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 15.41%. Comparing base (595f477) to head (c42c50d).
⚠️ Report is 405 commits behind head on main.

Files with missing lines	Patch %	Lines
...kmarks/explore-bookmark-legacy-data-transformer.ts	88.88%	0 Missing and 1 partial ⚠️
web-admin/src/features/bookmarks/utils.ts	88.88%	0 Missing and 1 partial ⚠️

❗ There is a different number of reports uploaded between BASE (595f477) and HEAD (c42c50d). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (595f477)	HEAD (c42c50d)
go	1	0

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #9739       +/-   ##
===========================================
- Coverage   44.98%   15.41%   -29.58%     
===========================================
  Files         726     2060     +1334     
  Lines       90172    62789    -27383     
  Branches        0    13271    +13271     
===========================================
- Hits        40562     9677    -30885     
- Misses      43484    51751     +8267     
+ Partials     6126     1361     -4765     

Flag	Coverage Δ
go	?
web-admin	4.56% <88.88%> (?)
web-common	17.11% <ø> (?)

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[nishantmonu51]

Temporarily closing to keep the first review wave focused. We will reopen this PR after #9730, #9731, and #9732 have merged. The branch is being retained.