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/gitea/client.go

@@ -753,6 +753,49 @@ func (c *Client) ApplyLabel(ctx context.Context, token, owner, repo string, inde
 	return nil
 }
 
+// ListCollaborators fetches the list of collaborators (users with access) for a repo.
+func (c *Client) ListCollaborators(ctx context.Context, token, owner, repo string) ([]string, error) {
+	path := fmt.Sprintf("/repos/%s/%s/collaborators?limit=50", owner, repo)
+	resp, err := c.doRequest(ctx, token, http.MethodGet, path, nil)
+	if err != nil {
+		return nil, fmt.Errorf("fetching collaborators: %w", err)
+	}
+	defer resp.Body.Close()
+
+	var users []struct {
+		Login string `json:"login"`
+	}
+	if err := json.NewDecoder(resp.Body).Decode(&users); err != nil {
+		return nil, fmt.Errorf("decoding collaborators: %w", err)
+	}
+
+	var logins []string
+	for _, u := range users {
+		logins = append(logins, u.Login)
+	}
+	return logins, nil
+}
+
+// AssignIssue sets the assignees on an issue.
+func (c *Client) AssignIssue(ctx context.Context, token, owner, repo string, index int64, assignees []string) error {
+	payload, err := json.Marshal(map[string]interface{}{
+		"assignees": assignees,
+	})
+	if err != nil {
+		return fmt.Errorf("marshaling assignees: %w", err)
+	}
+
+	path := fmt.Sprintf("/repos/%s/%s/issues/%d", owner, repo, index)
+	resp, err := c.doRequest(ctx, token, http.MethodPatch, path, strings.NewReader(string(payload)))
+	if err != nil {
+		return fmt.Errorf("assigning issue: %w", err)
+	}
+	resp.Body.Close()
+
+	c.InvalidateAll()
+	return nil
+}
+
 // SubmitReview submits a review on a pull request.
 func (c *Client) SubmitReview(ctx context.Context, token, owner, repo string, index int64, reviewType, body string) error {
 	payload := map[string]interface{}{