refactor: streamline PNG rendering by removing temporary file usage

[rajarshidattapy]

What

Fixes #4

renderCardToPNG (internal/cli/freeze.go) built freeze --execute "cat " + tmpfile, which broke bolna agents view <id> --png out.png two ways:

1. Windows: cat isn't on a standard Windows PATH, so --execute had nothing to run — --png couldn't work at all.
2. Spaces: freeze word-splits the --execute string, so any temp path with a space (e.g. C:\Users\John Doe\bolna-card-123.ansi) split into two args and failed — on any OS whose temp path contains a space.

Fix

Pipe the ANSI card to freeze's stdin instead of shelling out to cat. This is freeze's documented cat art.ansi | freeze usage — it reads and auto-detects ANSI on stdin. Both bugs die at the root: no cat dependency, and no temp file at all, so there's no path that could contain a space.

Also deletes the whole temp-file dance (os.CreateTemp, WriteString, Close, defer Remove) — −12 lines, smaller than the file-arg approach the issue suggested.

Verification

• go build ./..., go vet, go test ./... all pass.