Add line wrap toggle for editor and paste viewer (#130)

Shared client-side preference in localStorage 'palette-wrap', default
off. Toggle button on /new editor head and paste title bar, plus a Line
wrap checkbox in the settings menu; all read/write the same preference.
Active state uses the accent treatment like other toggles. No em dashes
in UI text.

internal/web/templates/paste.html

@@ -6,6 +6,7 @@
       <h1>{{if .Title}}{{.Title}}{{else}}Untitled paste{{end}}</h1>
       {{if .CustomSlug}}<span class="slug">/{{.CustomSlug}}</span>{{end}}
       <div class="spacer"></div>
+      <button type="button" class="iconbtn wrap-toggle" title="Toggle line wrap" aria-pressed="false">wrap</button>
       <a class="iconbtn" href="/raw/{{.ID}}">raw</a>
       <a class="iconbtn" href="#" id="copy-btn" onclick="copyContent(this); return false;">copy</a>
       {{if .DeletionToken}}<a class="iconbtn danger" href="#" onclick="redeem('{{.DeletionToken}}'); return false;">delete</a>{{end}}