feat: implement POST /pulls merge handler with merge button in PR detail

Add MergePull HTTP handler that calls the existing client.MergePull()
method, register the POST /pulls/{owner}/{repo}/{index}/merge route,
and add a merge button/form to the PR detail template gated on
Mergeable and open state. Supports merge, rebase, and squash styles.

- Handler returns HTMX fragment on HX-Request, redirect otherwise
- Error path returns inline error fragment for HTMX requests
- Add mock merge endpoint and 3 integration tests
- Merge button only shows when PR is mergeable and open

Closes leeworks-agents/gitea-mobile#229

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

internal/templates/pull_detail.html

@@ -33,6 +33,23 @@
   {{end}}
 </div>
 
+{{if and .Pull.Mergeable (eq .Pull.State "open")}}
+<div class="card" style="margin-top:1rem;" id="merge-section">
+  <h2>Merge Pull Request</h2>
+  <form hx-post="/pulls/{{.Pull.RepoOwner}}/{{.Pull.RepoName}}/{{.Pull.Number}}/merge" hx-target="#merge-section" hx-swap="innerHTML">
+    <div class="form-group">
+      <label for="merge-style">Merge Style</label>
+      <select id="merge-style" name="Do">
+        <option value="merge">Merge Commit</option>
+        <option value="rebase">Rebase</option>
+        <option value="squash">Squash</option>
+      </select>
+    </div>
+    <button type="submit" class="btn btn-primary" style="background-color:var(--accent-purple,#a371f7);">Merge PR</button>
+  </form>
+</div>
+{{end}}
+
 <div class="card" style="margin-top:1rem;">
   <h2>Submit Review</h2>
   <form hx-post="/pulls/{{.Pull.RepoOwner}}/{{.Pull.RepoName}}/{{.Pull.Number}}/review" hx-swap="outerHTML">