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.

main.go

@@ -2,12 +2,18 @@ package main
 
 import (
 	"fmt"
+	"io/fs"
 	"os"
 
 	"github.com/lerko/nib/cmd"
 )
 
 func main() {
+	webContent, err := fs.Sub(WebFS, "web")
+	if err == nil {
+		cmd.WebFS = webContent
+	}
+
 	if err := cmd.Execute(); err != nil {
 		fmt.Fprintln(os.Stderr, err)
 		os.Exit(1)