Add polished answer notifications (#1)

Sends a branded Resend email when a question receives its first answer, records accepted and failed sends, and adds collapsed answer editing with cancel behavior.

Co-authored-by: codegirl-007 <s.raide@gmail.com>

templates/register.html

@@ -9,6 +9,9 @@
     <label for="username">Username</label>
     <input id="username" name="username" type="text" required minlength="3" maxlength="20" pattern="[A-Za-z0-9_]+" autocomplete="username" autocapitalize="off" spellcheck="false" value="{{.Username}}">
     <p class="hint">3–20 letters, numbers, or underscores.</p>
+    <label for="email">Email</label>
+    <input id="email" name="email" type="email" required maxlength="254" autocomplete="email" value="{{.Email}}">
+    <p class="hint">We’ll email you when a plumber answers your question.</p>
     <label for="password">Password</label>
     <input id="password" name="password" type="password" required minlength="8" maxlength="72" autocomplete="new-password">
     <p class="hint">At least 8 characters (max 72 bytes).</p>