fix: replace cryptic SSH "missing server host" failure with actionable secrets validation

[Copilot]

The "Deploy to VPS" job was failing immediately with Error: missing server host because none of the four required secrets (VPS_HOST, VPS_USER, VPS_SSH_KEY, VPS_APP_DIR) were configured — appleboy/ssh-action receives empty strings and aborts with no guidance on what to set.

Changes

• Added Check required secrets pre-flight step — validates all four secrets are non-empty before the SSH action runs; fails with a structured error listing exactly which secrets are missing and where to add them:

  ::error::Required secrets not configured: VPS_HOST VPS_USER VPS_SSH_KEY VPS_APP_DIR
  Add these secrets in: Settings → Secrets and variables → Actions
  

• Quoted VPS_APP_DIR in cd — cd ${{ secrets.VPS_APP_DIR }} → cd "${{ secrets.VPS_APP_DIR }}" to correctly handle paths containing spaces.

Required follow-up

The repository owner must add these four secrets under Settings → Secrets and variables → Actions:

Secret	Value
VPS_HOST	VPS IP / hostname
VPS_USER	SSH login username
VPS_SSH_KEY	Private SSH key
VPS_APP_DIR	Absolute path to app on VPS (e.g. /home/ubuntu/apps/eventshare)