fix(ui): serve embedded SPA via ServeContent to avoid index.html redirect loop

http.FileServer canonicalizes /index.html -> "./", which for /ui/ produced a
301 redirect loop and made the control room unreachable. Serve embedded files
directly with http.ServeContent instead.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

cmd/oikos/main.go

@@ -1,14 +1,17 @@
 package main
 
 import (
+	"bytes"
 	"context"
 	"fmt"
+	"io"
 	"log/slog"
 	"net/http"
 	"os"
 	"os/signal"
 	"strings"
 	"syscall"
+	"time"
 
 	"github.com/dtoro/oikos/internal/config"
 	"github.com/dtoro/oikos/internal/db"
@@ -24,30 +27,38 @@ import (
 
 // uiHandler serves the control-room SPA from assets embedded at build time
 // (web/embed.go), with SPA fallback to index.html. Requests arrive as /ui/*;
-// the /ui prefix is stripped to index into the embedded dist/ tree.
+// the /ui prefix is stripped to index into the embedded dist/ tree. Files are
+// written via http.ServeContent (not http.FileServer) to avoid its
+// index.html -> "./" canonical redirect, which loops for /ui/.
 func uiHandler() http.Handler {
 	dist, err := web.DistFS()
 	if err != nil {
 		slog.Warn("ui: embedded assets unavailable", "error", err)
 		return http.NotFoundHandler()
 	}
-	fileServer := http.FileServer(http.FS(dist))
-	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		path := strings.TrimPrefix(strings.TrimPrefix(r.URL.Path, "/ui"), "/")
-		if path == "" {
-			path = "index.html"
+	serve := func(w http.ResponseWriter, r *http.Request, name string) bool {
+		f, err := dist.Open(name)
+		if err != nil {
+			return false
 		}
-		if f, err := dist.Open(path); err == nil {
-			f.Close()
-			r.URL.Path = "/" + path
-			fileServer.ServeHTTP(w, r)
+		defer f.Close()
+		data, err := io.ReadAll(f)
+		if err != nil {
+			return false
+		}
+		http.ServeContent(w, r, name, time.Time{}, bytes.NewReader(data))
+		return true
+	}
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		name := strings.TrimPrefix(strings.TrimPrefix(r.URL.Path, "/ui"), "/")
+		if name == "" {
+			name = "index.html"
+		}
+		if serve(w, r, name) {
 			return
 		}
 		// SPA fallback: serve index.html for unknown client-side routes.
-		if f, err := dist.Open("index.html"); err == nil {
-			f.Close()
-			r.URL.Path = "/index.html"
-			fileServer.ServeHTTP(w, r)
+		if serve(w, r, "index.html") {
 			return
 		}
 		http.NotFound(w, r)