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/Makefile

@@ -16,8 +16,11 @@ GOTEST=$(GOCMD) test
 GOGET=$(GOCMD) get
 GOMOD=$(GOCMD) mod
 
+# Version (overridable via: make build-linux VERSION=v1.2.3)
+VERSION ?= dev
+
 # Build flags
-LDFLAGS=-ldflags "-s -w"
+LDFLAGS = -ldflags "-s -w -X main.version=$(VERSION)"
 
 all: clean build