feat(all): add public GitHub repository deployment without GitHub App

Allow users to deploy MCP servers from public GitHub repositories by
pasting a URL directly, without installing the DeployStack GitHub App.

Backend:
- Refactor DeploymentValidationService into validate(), validatePublic(),
  and shared _validateWithOctokit() private method
- Add GET /deploy/public/branches endpoint for unauthenticated branch listing
- Add deployment_source field to validate and deploy request schemas
- Add deployment_source column to mcpServers table (migration 0028)
- Return 404 instead of 500 from github-token endpoint when no credentials found

Satellite:
- Make token parameter optional in downloadRepository()
- Handle missing GitHub token gracefully in prepareDeployment()
- Log 404 token responses as info instead of error in backend-client

Frontend:
- Add shadcn-vue Tabs component
- Rewrite SelectRepositoryStep with two tabs: GitHub App and Public Repository
- Add getPublicBranches() to DeploymentService
- Pass deployment_source through validate and deploy API calls

Author: Lasim

package-lock.json

@@ -0,0 +1 @@
+ALTER TABLE "mcpServers" ADD COLUMN "deployment_source" text;