feat: add close/reopen action to PR detail view

Add POST /pulls/{owner}/{repo}/{index}/state handler that reuses the
existing SetIssueState Gitea API call (PRs share the issues state
endpoint). The PR detail template now shows a Close PR / Reopen PR
button with HTMX for seamless state toggling without full page reload.
Also fixes the state badge to use the correct CSS class when a PR is
closed.

Closes leeworks-agents/gitea-mobile#91

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/templates/pull_detail.html

@@ -4,7 +4,7 @@
 <div class="card">
   <div class="card-meta">
     <span class="type-badge type-pull">PR</span>
-    <span class="state-open">{{.Pull.State}}</span>
+    {{if eq .Pull.State "closed"}}<span class="state-closed">{{.Pull.State}}</span>{{else}}<span class="state-open">{{.Pull.State}}</span>{{end}}
     <span>{{.Pull.RepoOwner}}/{{.Pull.RepoName}} #{{.Pull.Number}}</span>
     {{range .Pull.Labels}}
     <span class="label" style="color:#{{.Color}};border:1px solid #{{.Color}}">{{.Name}}</span>
@@ -15,6 +15,17 @@
     <span class="diff-del">-{{.Pull.Deletions}}</span>
     {{if .Pull.Mergeable}}<span style="color:var(--accent-green);">Mergeable</span>{{end}}
   </div>
+  <div class="card-meta" style="margin-top:0.5rem;">
+    <span id="state-section">
+      {{if eq .Pull.State "closed"}}
+      <span class="state-closed" id="pull-state">{{.Pull.State}}</span>
+      <button class="btn btn-secondary" hx-post="/pulls/{{.Pull.RepoOwner}}/{{.Pull.RepoName}}/{{.Pull.Number}}/state" hx-vals='{"state":"open"}' hx-target="#state-section" hx-swap="innerHTML">Reopen PR</button>
+      {{else}}
+      <span class="state-open" id="pull-state">{{.Pull.State}}</span>
+      <button class="btn btn-danger" hx-post="/pulls/{{.Pull.RepoOwner}}/{{.Pull.RepoName}}/{{.Pull.Number}}/state" hx-vals='{"state":"closed"}' hx-target="#state-section" hx-swap="innerHTML">Close PR</button>
+      {{end}}
+    </span>
+  </div>
   {{if .RenderedBody}}
   <div class="card-body markdown-body">{{.RenderedBody}}</div>
   {{else if .Pull.Body}}