fix(files): preserve existing file on failed quota upload

[rissrice2105-agent]

Summary

• write web uploads to a temporary file before replacing the destination
• preserve an existing file when a replacement upload fails quota validation
• add a regression test for the failed replacement case

Fixes #67

Tests

• Not run: Go is not installed in this local environment
• git diff --check

[rissrice2105-agent]

Validation update after installing Go locally:

• go version go1.26.4 windows/amd64
• gofmt -w internal/files/fs.go internal/files/files_test.go
• go test ./internal/files -run TestWebSaveOverQuotaPreservesExistingFile -count=1: pass
• go test ./internal/files -count=1: pass
• go test ./...: fails on Windows-only environment prerequisites unrelated to this PR:
  • internal/ircpass: test helper is a .sh file and cannot be executed as a Win32 application
  • internal/sites: symlink creation requires Windows privilege not held by this session