ua-labels: derive source from User-Agent, persist peerID in localStorage

Server now parses the User-Agent header on /api/send when no explicit
source is provided, producing labels like 'iphone-safari',
'macos-chrome', 'windows-firefox' so the feed shows where messages
came from at a glance.

Browser-side peerID is now stored in localStorage instead of being
freshly random on each page load — refreshing or reopening the tab
keeps the same identity for the same browser profile.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

server/main.go

@@ -31,11 +31,45 @@ import (
 	"sync"
 	"time"
 
+	"github.com/mileusna/useragent"
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promauto"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 )
 
+// labelFromUA returns a short, human-friendly platform label like
+// "iphone-safari", "macos-chrome", "windows-firefox". Falls back to "web".
+func labelFromUA(uaStr string) string {
+	ua := useragent.Parse(uaStr)
+	platform := strings.ToLower(ua.OS)
+	switch {
+	case ua.IsIOS() && strings.Contains(strings.ToLower(ua.Device), "ipad"):
+		platform = "ipad"
+	case ua.IsIOS():
+		platform = "iphone"
+	case ua.IsAndroid():
+		platform = "android"
+	case ua.IsMacOS():
+		platform = "macos"
+	case ua.IsWindows():
+		platform = "windows"
+	case ua.IsLinux():
+		platform = "linux"
+	}
+	browser := strings.ToLower(ua.Name)
+	browser = strings.ReplaceAll(browser, " ", "")
+	if platform == "" && browser == "" {
+		return "web"
+	}
+	if browser == "" {
+		return platform
+	}
+	if platform == "" {
+		return browser
+	}
+	return platform + "-" + browser
+}
+
 //go:embed web
 var webFS embed.FS
 
@@ -251,7 +285,7 @@ func main() {
 		if m.Source == "" {
 			m.Source = r.Header.Get("X-Tether-Source")
 			if m.Source == "" {
-				m.Source = "web"
+				m.Source = labelFromUA(r.Header.Get("User-Agent"))
 			}
 		}
 		m.TS = time.Now().UnixMilli()