Fix #214-ui: add page <title> to every rendered page

layout.html now emits <title>{{ pageTitle . }} · Palette</title>; the
pageTitle template func maps the Page field to a label (New paste,
Public pastes, Saved, Settings, paste title or 'Paste <id>', Unlock,
Can, Not found).

internal/web/templates/layout.html

@@ -1,5 +1,6 @@
 {{define "head"}}
 <meta charset="utf-8">
+<title>{{ pageTitle . }} · Palette</title>
 <meta name="viewport" content="width=device-width, initial-scale=1">
 <link rel="stylesheet" href="/static/app.css">
 <link rel="icon" type="image/svg+xml" href="/static/favicon.svg">