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/deployment/corrosion-companion.service

@@ -0,0 +1,28 @@
+[Unit]
+Description=Corrosion Companion Agent
+Documentation=https://corrosionmgmt.com
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=simple
+User=rust
+Group=rust
+WorkingDirectory=/opt/corrosion
+ExecStart=/opt/corrosion/corrosion-companion-linux-amd64
+Restart=on-failure
+RestartSec=5
+StartLimitInterval=60
+StartLimitBurst=3
+
+# Environment
+EnvironmentFile=/opt/corrosion/.env
+
+# Security hardening
+NoNewPrivileges=true
+ProtectSystem=strict
+ReadWritePaths=/opt/corrosion /home/rust
+ProtectHome=read-only
+
+[Install]
+WantedBy=multi-user.target