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/api/router.go

@@ -1,6 +1,8 @@
 package api
 
 import (
+	"net/http"
+
 	"github.com/gin-gonic/gin"
 	"github.com/llamalink/llamalink/internal/api/handlers"
 	"github.com/llamalink/llamalink/internal/api/middleware"
@@ -8,6 +10,7 @@ import (
 	"github.com/llamalink/llamalink/internal/config"
 	"github.com/llamalink/llamalink/internal/llama"
 	"github.com/llamalink/llamalink/internal/quota"
+	"github.com/llamalink/llamalink/internal/web"
 	"gorm.io/gorm"
 )
 
@@ -73,5 +76,32 @@ func New(cfg *config.Config, db *gorm.DB, llamaManager *llama.Manager) *gin.Engi
 	usage.GET("", usageHandler.GetCurrentKeyUsage)
 	usage.GET("/:key_id", usageHandler.GetUsage)
 
+	// Admin SPA (static files with embedded frontend)
+	r.GET("/assets/*filepath", func(c *gin.Context) {
+		filepath := c.Param("filepath")
+		data, err := web.ServeAsset(filepath)
+		if err != nil {
+			c.String(http.StatusNotFound, "asset not found")
+			return
+		}
+		c.Data(http.StatusOK, http.DetectContentType(data), data)
+	})
+	r.GET("/admin", func(c *gin.Context) {
+		index, err := web.Index()
+		if err != nil {
+			c.String(http.StatusInternalServerError, "index.html not found")
+			return
+		}
+		c.Data(http.StatusOK, "text/html; charset=utf-8", index)
+	})
+	r.GET("/admin/*filepath", func(c *gin.Context) {
+		index, err := web.Index()
+		if err != nil {
+			c.String(http.StatusInternalServerError, "index.html not found")
+			return
+		}
+		c.Data(http.StatusOK, "text/html; charset=utf-8", index)
+	})
+
 	return r
 }