Add PWA installable support for tangled

/pwa-manifest.json is defined in state.go and is a plain json object
the link rel has to be added to all html heads

appview/pages/templates/layouts/base.html

···
             <link rel="preconnect" href="https://avatar.tangled.sh" />
             <link rel="preconnect" href="https://camo.tangled.sh" />
 
+            <!-- pwa manifest -->
+            <link rel="manifest" href="/static/pwa-manifest.json" />
+
             <!-- preload main font -->
             <link rel="preload" href="/static/fonts/InterVariable.woff2" as="font" type="font/woff2" crossorigin />
 

appview/pages/templates/user/completeSignup.html

···
                 content="complete your signup for tangled"
             />
             <script src="/static/htmx.min.js"></script>
+            <link rel="manifest" href="/static/pwa-manifest.json" />
             <link
                 rel="stylesheet"
                 href="/static/tw.css?{{ cssContentHash }}"

appview/pages/templates/user/login.html

···
             <meta property="og:url" content="https://tangled.org/login" />
             <meta property="og:description" content="login to for tangled" />
             <script src="/static/htmx.min.js"></script>
+            <link rel="manifest" href="/static/pwa-manifest.json" />
             <link rel="stylesheet" href="/static/tw.css?{{ cssContentHash }}" type="text/css" />
             <title>login &middot; tangled</title>
         </head>

appview/pages/templates/user/signup.html

···
             <meta property="og:url" content="https://tangled.org/signup" />
             <meta property="og:description" content="sign up for tangled" />
             <script src="/static/htmx.min.js"></script>
+            <link rel="manifest" href="/static/pwa-manifest.json" />
             <link rel="stylesheet" href="/static/tw.css?{{ cssContentHash }}" type="text/css" />
             <title>sign up &middot; tangled</title>
 

appview/state/router.go

···
 	router.Use(middleware.TryRefreshSession())
 	router.Get("/favicon.svg", s.Favicon)
 	router.Get("/favicon.ico", s.Favicon)
+	router.Get("/pwa-manifest.json", s.PWAManifest)
 
 	userRouter := s.UserRouter(&middleware)
 	standardRouter := s.StandardRouter(&middleware)

appview/state/state.go

···
 	s.pages.Favicon(w)
 }
 
+// https://developer.mozilla.org/en-US/docs/Web/Progressive_web_apps/Manifest
+const manifestJson = `{
+  "name": "tangled",
+  "description": "tightly-knit social coding.",
+  "icons": [
+    {
+      "src": "/favicon.svg",
+      "sizes": "144x144"
+    }
+  ],
+  "start_url": "/",
+  "id": "org.tangled",
+
+  "display": "standalone",
+  "background_color": "#111827",
+  "theme_color": "#111827"
+}`
+
+func (p *State) PWAManifest(w http.ResponseWriter, r *http.Request) {
+	w.Header().Set("Content-Type", "application/json")
+	w.Write([]byte(manifestJson))
+}
+
 func (s *State) TermsOfService(w http.ResponseWriter, r *http.Request) {
 	user := s.oauth.GetUser(r)
 	s.pages.TermsOfService(w, pages.TermsOfServiceParams{