feat: modernize codebase and complete API surface

- Update module path from prologic/pastebin to taigrr/pastebin
- Replace go.rice with stdlib embed for templates and static assets
- Replace httprouter with stdlib net/http ServeMux (Go 1.22+)
- Replace namsral/flag with cobra + fang for CLI argument parsing
- Fix inverted TLS insecure logic in client (was negating the flag)
- Close HTTP response body in client
- Use base64 URLEncoding instead of StdEncoding for paste IDs
- Add comprehensive handler tests for all endpoints
- Add client package tests
- Add utils tests for RandomString
- Remove templates.go (replaced by embed)
- Remove unused dependencies (go-metrics, stats, logger, httprouter, go.rice)

templates/base.html

@@ -15,7 +15,7 @@
       </section>
       <section class="navbar-section"></section>
     </header>
-    {{template "content" .}}
+    {{if .}}{{template "view_content" .}}{{else}}{{template "index_content"}}{{end}}
   </section>
 </body>
 </html>

templates/index.html

@@ -1,4 +1,4 @@
-{{define "content"}}
+{{define "index_content"}}
 <section class="container">
   <div class="columns">
     <div class="column">

templates/view.html

@@ -1,4 +1,4 @@
-{{define "content"}}
+{{define "view_content"}}
 <pre class="code">
   <code>{{.Blob}}</code>
 </pre>
@@ -16,7 +16,7 @@
           </a>
           <ul class="menu">
             <li class="menu-item">
-	      <form action="/delete/{{.UUID}}" method="POST">
+              <form action="/delete/{{.UUID}}" method="POST">
                 <button type="submit">
                   <span href="#dropdowns">
                     <i class="icon icon-delete"></i>