fix(security): redesign password reset tokens using crypto/rand with server-side storage

- Replace SHA512-based deterministic token with 32-byte crypto/rand token
- Store tokens server-side with 1-hour expiry and single-use semantics
- Remove genToken (previously broken due to time.Add immutability bug)
- Add CSRF double-submit cookie protection to change/lost/reset forms
- Remove token from form action URL (use hidden fields only, POST body)
- Add MailFrom field and SMTP_FROM env var for configurable sender address
- Add SMTP_PASSWORD_FILE env var for secure SMTP password loading
- Add PUBLIC_URL env var and --public-url flag for configurable reset link domain
- Use generic error messages in handlers to avoid information disclosure

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

static/reset.html

@@ -1,8 +1,9 @@
 {{template "header"}}
   <h1 class="display-4">Forgot your password? <small class="text-muted">Define a new one!</small></h1>
 
-  <form method="post" action="reset?l={{ .login }}&t={{ .token }}">
+  <form method="post" action="reset">
     {{if .error}}<div class="alert alert-danger" role="alert">{{.error}}</div>{{end}}
+    <input type="hidden" name="csrf_token" value="{{ .csrf_token }}">
     <div class="form-group">
       <input required="" class="form-control" id="input_0" type="text" placeholder="Email" value="{{ .login }}" disabled="">
     </div>