Add embedded Vue SPA admin panel with go:embed

This is the complete fix for the missing /admin/ route and frontend serving:

Backend:
- internal/web/web.go: new package with go:embed for web/dist/
- internal/api/router.go: add routes for /admin/, /admin/*, /assets/*
- internal/db/db.go: fix SQLite DSN parsing (sqlite:///path -> path)

Build system:
- Makefile: new 'embed-prep' target copies web/dist to internal/web/dist
- make build now runs embed-prep -> frontend/build automatically

Deployment:
- deploy/llamalink.service: remove invalid --host/--port flags,
  add EnvironmentFile=/etc/llamalink/env

Verified:
- /health returns 200
- /admin/ serves Vue SPA HTML
- /assets/* serves CSS and JS files from embedded FS
- sqlite:///./llamalink.db works correctly

internal/db/db.go

@@ -13,9 +13,12 @@ import (
 )
 
 func Open(cfg *config.Config) (*gorm.DB, error) {
-	dsn := strings.TrimPrefix(cfg.DatabaseURL, "sqlite://")
-	if dsn == cfg.DatabaseURL {
-		dsn = cfg.DatabaseURL
+	dsn := cfg.DatabaseURL
+	if strings.HasPrefix(dsn, "sqlite://") {
+		dsn = strings.TrimPrefix(dsn, "sqlite://")
+		if strings.HasPrefix(dsn, "/") {
+			dsn = dsn[1:]
+		}
 	}
 
 	gormConfig := &gorm.Config{