fix(desktop): resolve base.css from import.meta.url, not process.cwd()

[h4yfans]

Summary

• theme-tokens.test.ts located base.css relative to process.cwd(), which is apps/desktop when vitest runs there but the repo root when invoked with an explicit --config path from the root — the documented way to run a targeted test in a fresh worktree. That threw ENOENT for a reason unrelated to the test's subject.
• Fixed by resolving from import.meta.url instead, which jsdom gives a real file: URL regardless of invocation cwd.
• Grepped process.cwd() under apps/desktop and found the same cwd-dependent file-resolution bug in five more places: blocknote-text-color-css.test.ts, excalidraw-context-menu-css.test.ts, mind-map-library-contract.test.ts, review-ui.test.tsx, and the shared helper tests/utils/contrast.ts (outside src, so it wasn't in the issue's stated grep scope but hits the same bug). All fixed the same way.
• Left the remaining process.cwd() hits alone: tailwind-arbitrary-css-vars.test.ts already falls back between both cwds, terminal-command.test.ts and paths.test.ts use cwd for synthetic/output paths rather than resolving a fixed source file, and check-cert-hashes.ts is a script that defaults an arg, not a test.

Test plan

• pnpm exec vitest run --config config/vitest.config.ts --project renderer <paths> from apps/desktop — 5 files, 81 tests passed
• ./node_modules/.bin/vitest run --config apps/desktop/config/vitest.config.ts --project renderer <paths> from the repo root — 5 files, 81 tests passed

Closes #1957

[github-actions]

React Doctor found no new issues. 🎉

_{Reviewed by React Doctor for commit 4794b4a.}

[codecov]

Codecov Report

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