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/web/web.go

@@ -0,0 +1,25 @@
+package web
+
+import (
+	"embed"
+	"io/fs"
+	"net/http"
+	"path"
+)
+
+//go:embed all:dist
+var distFS embed.FS
+
+func FileSystem() http.FileSystem {
+	sub, _ := fs.Sub(distFS, "dist")
+	return http.FS(sub)
+}
+
+func Index() ([]byte, error) {
+	return distFS.ReadFile("dist/index.html")
+}
+
+func ServeAsset(filepath string) ([]byte, error) {
+	fullPath := path.Join("dist/assets", filepath)
+	return distFS.ReadFile(fullPath)
+}