feat: Companion agent download in ServerView + Gitea CI pipeline fix

Frontend:
- Add Companion Agent card to ServerView (status, download links, setup instructions)
- Shows agent connection status, last heartbeat, license key for copy
- Download buttons for Linux/Windows amd64 from Gitea releases

CI/CD:
- Fix build-companion.yml: replace actions/github-script with Gitea API curl
- Inject version from git tag via ldflags (-X main.version)
- Add VERSION variable to Makefile with ldflags injection
- Change main.go version from const to var for ldflags compatibility

Deployment:
- Add systemd service file (deployment/corrosion-companion.service)
- Add .gitignore for bin/ (binaries should come from CI, not repo)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

companion-agent/cmd/agent/main.go

@@ -32,7 +32,7 @@ type Config struct {
 	LogLevel          string `envconfig:"LOG_LEVEL" default:"info"`
 }
 
-const version = "1.0.0"
+var version = "dev"
 
 func main() {
 	log.SetFlags(log.LstdFlags | log.Lshortfile)