feat(P1): Reach signup handler + templates + tests (REQ-040, G-026)

web/handlers/reach.go: GET /reach (list), GET /reach/new (form),
GET /reach/{id} (detail), POST /reach (atomic Reach+Stash create
per D-071, redirect 302). Labels "Create a Reach" (not the banned
legacy word). G-027 validation (non-empty, <=128, no path separators,
no template syntax). 3 Reach templates extend base.html.
reach_test.go: httptest for all 4 routes + atomic create + 400/409
error paths + rendered-HTML lexicon check on BOTH 200 and error bodies
(G-026). handlers/server.go: clone-per-page template pattern (avoids
content-block collision across pages). Coverage 81.2% on web/handlers.

---ci---
project: oy
phase: 1
milestone: v0.6
status: execute
---/ci---

web/server.go

@@ -1,10 +1,12 @@
 package main
 
 import (
-	"html/template"
 	"log"
 	"net/http"
 	"os"
+
+	"github.com/oy/openyield/web/handlers"
+	"github.com/oy/openyield/web/store"
 )
 
 func runServer() {
@@ -13,29 +15,28 @@ func runServer() {
 		port = "8080"
 	}
 
-	tmpl, err := template.ParseGlob("web/templates/*.html")
-	if err != nil {
-		log.Fatalf("parse templates: %v", err)
-	}
-
 	mux := http.NewServeMux()
 
+	srv, err := handlers.New(store.NewStore(), "web/templates")
+	if err != nil {
+		log.Fatalf("init handlers: %v", err)
+	}
+	srv.Register(mux)
+
+	// Home page (rendered via the handlers' page machinery too).
 	mux.HandleFunc("GET /", func(w http.ResponseWriter, r *http.Request) {
 		if r.URL.Path != "/" {
 			http.NotFound(w, r)
 			return
 		}
-		w.Header().Set("Content-Type", "text/html; charset=utf-8")
-		if err := tmpl.ExecuteTemplate(w, "home.html", nil); err != nil {
-			log.Printf("render home: %v", err)
-		}
+		srv.RenderHome(w, nil)
 	})
 
 	mux.Handle("GET /static/", http.StripPrefix("/static/", http.FileServer(http.Dir("web/static"))))
 
-	srv := &http.Server{Addr: ":" + port, Handler: mux}
+	server := &http.Server{Addr: ":" + port, Handler: mux}
 	log.Printf("OpenYield web on :%s", port)
-	if err := srv.ListenAndServe(); err != nil {
+	if err := server.ListenAndServe(); err != nil {
 		log.Fatalf("server: %v", err)
 	}
 }