Switch PyInstaller backend build from --onefile to --onedir

[Copilot]

--onefile extracts the entire bundled archive to a temp directory on every launch, causing significant startup latency. Switching to --onedir eliminates that extraction step — the EXE runs directly from its directory.

PyInstaller spec (stasis-backend.spec)

• Removed a.binaries and a.datas from EXE() args (onefile pattern)
• Added COLLECT() block with a.binaries and a.datas (onedir pattern)
• Removed runtime_tmpdir (onefile-only)

Build script (build.ps1)

• $DistDir / $DistExe now point into dist\stasis-backend\ (onedir output)
• Copy step uses Copy-Item -Recurse to copy the full directory to frontend\src-tauri\bin\stasis-backend\
• Versioned release artifact is now a zip (Compress-Archive) instead of a renamed .exe

Tauri bundling

• tauri.conf.json resources changed from single-file mapping to glob: "bin/stasis-backend/**/*"
• main.rs backend resolution path updated: bin/stasis-backend.exe → bin/stasis-backend/stasis-backend.exe

GitHub workflow (.github/workflows/build-and-release.yml)

• Release artifact upload pattern updated from dist/stasis-backend-v*.exe to dist/stasis-backend-v*.zip

Docs

• README, docs/developer-guide.md, and docs/architecture.md updated to reflect the new onedir output paths and artifact format.

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