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/dist/assets/plus-Bfa9PGWP.js

@@ -0,0 +1,6 @@
+import{c as e}from"./createLucideIcon-CUrbWv4G.js";/**
+ * @license lucide-vue-next v0.460.0 - ISC
+ *
+ * This source code is licensed under the ISC license.
+ * See the LICENSE file in the root directory of this source tree.
+ */const a=e("PlusIcon",[["path",{d:"M5 12h14",key:"1ays0h"}],["path",{d:"M12 5v14",key:"s699le"}]]);export{a as P};