fix(release): install Bun for desktop packaging

[wolfiesch]

Summary

• install the pinned Bun compiler in Linux and macOS release build jobs
• reuse the exact setup action and Bun version already exercised by main CI
• add a regression test that requires Bun setup before each desktop package command

Evidence

• release run 30063940295 passed verification, CI authority, and Android, then both desktop jobs failed at bun build --compile with bun: not found
• mise exec node@24.13.1 -- pnpm exec node --test scripts/packaging.test.mjs
• mise exec node@24.13.1 -- pnpm test:tooling (323 passed, 2 skipped)
• mise exec node@24.13.1 -- pnpm check
• mise exec node@24.13.1 -- pnpm lint