feat(web): add vanilla JS/CSS SPA with embed.FS

Stream view with date grouping, card view sorted by usage, capture
bar with client-side grammar parsing, tag rail filter, detail pane
with card affordances (template slot fill, checklist toggle, link
open), promote modal with auto-detect, keyboard shortcuts (j/k/n/p/
Enter/dd/1/2). Dark theme, responsive layout. Embedded in Go binary.

internal/api/router.go

@@ -1,6 +1,7 @@
 package api
 
 import (
+	"io/fs"
 	"net/http"
 
 	"github.com/go-chi/chi/v5"
@@ -8,7 +9,7 @@ import (
 	"github.com/lerko/nib/internal/db"
 )
 
-func NewRouter(store *db.Store, devMode bool) chi.Router {
+func NewRouter(store *db.Store, devMode bool, webFS ...fs.FS) chi.Router {
 	r := chi.NewRouter()
 
 	r.Use(middleware.Logger)
@@ -32,9 +33,28 @@ func NewRouter(store *db.Store, devMode bool) chi.Router {
 		r.Get("/tags", listTags(store))
 	})
 
+	if len(webFS) > 0 && webFS[0] != nil {
+		r.Get("/*", spaHandler(webFS[0]))
+	}
+
 	return r
 }
 
+func spaHandler(fsys fs.FS) http.HandlerFunc {
+	fileServer := http.FileServer(http.FS(fsys))
+	indexHTML, _ := fs.ReadFile(fsys, "index.html")
+
+	return func(w http.ResponseWriter, r *http.Request) {
+		path := r.URL.Path
+		if path == "/" || path == "/cards" {
+			w.Header().Set("Content-Type", "text/html; charset=utf-8")
+			w.Write(indexHTML)
+			return
+		}
+		fileServer.ServeHTTP(w, r)
+	}
+}
+
 func jsonContentType(next http.Handler) http.Handler {
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		w.Header().Set("Content-Type", "application/json")