fix(auth): use Tauri runtime for desktop OAuth

[Harry19081]

Summary

• detect the Tauri runtime in both development and release builds
• use the desktop OAuth callback and external-browser handoff whenever hosted by Tauri
• preserve the HTTP callback for web builds

Split from #442.

Validation

• pnpm exec vitest run src/config/serviceAuth.test.ts — 2 tests passed
• targeted ESLint and Prettier via the pre-commit hook
• scoped TypeScript pre-commit check

[Harry19081]

CI note: the frontend job currently stops during typecheck on unchanged base code at src/modules/shared/layouts/blocks/ScrollPreservation.test.ts:17 (No overload matches this call). This PR has no diff in that file, and the same error is reproduced across the independent split branches. Targeted validation for this PR is recorded in the description; rebase/rerun after the develop baseline fix lands.