feat: add Assign action to issue detail view

Add the ability to assign users to issues from the mobile app:
- New ListCollaborators client method fetches repo collaborators
- New AssignIssue client method sets assignees via PATCH API
- New POST /issues/{owner}/{repo}/{index}/assignees handler
- Assignee dropdown populated with repo collaborators in issue detail
- HTMX inline response confirms assignment without page reload

Closes leeworks-agents/gitea-mobile#50

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

internal/templates/issue_detail.html

@@ -51,6 +51,18 @@
 
 <div class="card" style="margin-top:1rem;">
   <h2>Actions</h2>
+  {{if .Collaborators}}
+  <form hx-post="/issues/{{.Issue.RepoOwner}}/{{.Issue.RepoName}}/{{.Issue.Number}}/assignees" hx-swap="outerHTML" style="margin-bottom:0.5rem;">
+    <div class="filter-bar" style="margin-bottom:0.5rem;">
+      <select name="assignee">
+        {{range .Collaborators}}
+        <option value="{{.}}">{{.}}</option>
+        {{end}}
+      </select>
+      <button type="submit" class="btn btn-secondary" style="width:auto;padding:0.5rem 1rem;">Assign</button>
+    </div>
+  </form>
+  {{end}}
   <form hx-post="/issues/{{.Issue.RepoOwner}}/{{.Issue.RepoName}}/{{.Issue.Number}}/labels" hx-swap="outerHTML" style="margin-bottom:0.5rem;">
     <div class="filter-bar" style="margin-bottom:0.5rem;">
       <select name="label_id">