Add Arc browser support on macOS

[YNDmitry]

Summary

Adds support for the Arc browser on macOS.

Arc stores cookies in a Chromium-style profile under:

~/Library/Application Support/Arc/User Data

and uses its own macOS keychain entry:

• account: Arc
• service/name: Arc Safe Storage

This PR adds:

• a new browser/arc package
• automatic Arc finder registration through browser/all
• Arc safe storage documentation in internal/chrome/README.md
• a small finder fix to skip non-existent cookie DB paths before yielding stores

Verification

Tested with:

go test ./...
go test ./browser/arc ./browser/all
go run ./cmd/kooky -b arc -j

The local smoke test successfully detected and decrypted cookies from an installed Arc profile on macOS.

Notes

This change currently adds automatic Arc profile discovery on macOS, which is the platform I was able to verify locally.

Arc on this machine stores cookies in:

~/Library/Application Support/Arc/User Data/Default/Cookies

and not in Default/Network/Cookies, so the finder now skips missing candidate paths instead of yielding a broken store first.