Add CSRF protection to state-changing POST endpoints

Add gorilla/csrf middleware to protect all HTML-serving routes against
cross-site request forgery attacks. The webhook endpoint is excluded
since it uses secret-based authentication.

Changes:
- Add gorilla/csrf v1.7.3 dependency
- Add CSRF() middleware method using session secret as key
- Apply CSRF middleware to all HTML route groups in routes.go
- Pass CSRF token to all templates via addGlobals helper
- Add {{ .CSRFField }} / {{ $.CSRFField }} hidden inputs to all forms

Closes #11

templates/app_detail.html

@@ -35,6 +35,7 @@
         <div class="flex gap-3">
             <a href="/apps/{{.App.ID}}/edit" class="btn-secondary">Edit</a>
             <form method="POST" action="/apps/{{.App.ID}}/deploy" class="inline" @submit="submitDeploy()">
+                {{ .CSRFField }}
                 <button type="submit" class="btn-success" x-bind:disabled="deploying" x-bind:class="{ 'opacity-50 cursor-not-allowed': deploying }">
                     <span x-text="deploying ? 'Deploying...' : 'Deploy Now'"></span>
                 </button>
@@ -106,6 +107,7 @@
                         <td class="font-mono text-gray-500">{{.Value}}</td>
                         <td class="text-right">
                             <form method="POST" action="/apps/{{$.App.ID}}/env/{{.ID}}/delete" class="inline" x-data="confirmAction('Delete this environment variable?')" @submit="confirm($event)">
+                {{ .CSRFField }}
                                 <button type="submit" class="text-error-500 hover:text-error-700 text-sm">Delete</button>
                             </form>
                         </td>
@@ -116,6 +118,7 @@
         </div>
         {{end}}
         <form method="POST" action="/apps/{{.App.ID}}/env" class="flex flex-col sm:flex-row gap-2">
+                {{ .CSRFField }}
             <input type="text" name="key" placeholder="KEY" required class="input flex-1 font-mono text-sm">
             <input type="text" name="value" placeholder="value" required class="input flex-1 font-mono text-sm">
             <button type="submit" class="btn-primary">Add</button>
@@ -149,6 +152,7 @@
                         <td class="font-mono text-gray-500">{{.Value}}</td>
                         <td class="text-right">
                             <form method="POST" action="/apps/{{$.App.ID}}/labels/{{.ID}}/delete" class="inline" x-data="confirmAction('Delete this label?')" @submit="confirm($event)">
+                {{ .CSRFField }}
                                 <button type="submit" class="text-error-500 hover:text-error-700 text-sm">Delete</button>
                             </form>
                         </td>
@@ -158,6 +162,7 @@
             </table>
         </div>
         <form method="POST" action="/apps/{{.App.ID}}/labels" class="flex flex-col sm:flex-row gap-2">
+                {{ .CSRFField }}
             <input type="text" name="key" placeholder="label.key" required class="input flex-1 font-mono text-sm">
             <input type="text" name="value" placeholder="value" required class="input flex-1 font-mono text-sm">
             <button type="submit" class="btn-primary">Add</button>
@@ -192,6 +197,7 @@
                         </td>
                         <td class="text-right">
                             <form method="POST" action="/apps/{{$.App.ID}}/volumes/{{.ID}}/delete" class="inline" x-data="confirmAction('Delete this volume mount?')" @submit="confirm($event)">
+                {{ .CSRFField }}
                                 <button type="submit" class="text-error-500 hover:text-error-700 text-sm">Delete</button>
                             </form>
                         </td>
@@ -202,6 +208,7 @@
         </div>
         {{end}}
         <form method="POST" action="/apps/{{.App.ID}}/volumes" class="flex flex-col sm:flex-row gap-2 items-end">
+                {{ .CSRFField }}
             <div class="flex-1 w-full">
                 <input type="text" name="host_path" placeholder="/host/path" required class="input font-mono text-sm">
             </div>
@@ -244,6 +251,7 @@
                         </td>
                         <td class="text-right">
                             <form method="POST" action="/apps/{{$.App.ID}}/ports/{{.ID}}/delete" class="inline" x-data="confirmAction('Delete this port mapping?')" @submit="confirm($event)">
+                {{ .CSRFField }}
                                 <button type="submit" class="text-error-500 hover:text-error-700 text-sm">Delete</button>
                             </form>
                         </td>
@@ -254,6 +262,7 @@
         </div>
         {{end}}
         <form method="POST" action="/apps/{{.App.ID}}/ports" class="flex flex-col sm:flex-row gap-2 items-end">
+                {{ .CSRFField }}
             <div class="flex-1 w-full">
                 <label class="block text-xs text-gray-500 mb-1">Host (external)</label>
                 <input type="text" name="host_port" placeholder="8080" required pattern="[0-9]+" class="input font-mono text-sm">
@@ -339,6 +348,7 @@
         <h2 class="text-lg font-medium text-error-700 mb-4">Danger Zone</h2>
         <p class="text-error-600 text-sm mb-4">Deleting this app will remove all configuration and deployment history. This action cannot be undone.</p>
         <form method="POST" action="/apps/{{.App.ID}}/delete" x-data="confirmAction('Are you sure you want to delete this app? This action cannot be undone.')" @submit="confirm($event)">
+                {{ .CSRFField }}
             <button type="submit" class="btn-danger">Delete App</button>
         </form>
     </div>