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/handlers/handlers.go

@@ -41,6 +41,7 @@ func (h *Handler) RegisterRoutes(mux *http.ServeMux) {
 	mux.HandleFunc("GET /issues/new", h.NewIssue)
 	mux.HandleFunc("POST /issues", h.CreateIssue)
 	mux.HandleFunc("POST /issues/{owner}/{repo}/{index}/labels", h.ApplyLabels)
+	mux.HandleFunc("POST /issues/{owner}/{repo}/{index}/assignees", h.AssignIssue)
 	mux.HandleFunc("POST /issues/{owner}/{repo}/{index}/close", h.CloseIssue)
 	mux.HandleFunc("POST /issues/{owner}/{repo}/{index}/state", h.SetIssueState)
 	mux.HandleFunc("POST /issues/{owner}/{repo}/{index}/comments", h.AddComment)
@@ -429,6 +430,12 @@ func (h *Handler) IssueDetail(w http.ResponseWriter, r *http.Request) {
 		labels = nil
 	}
 
+	collaborators, err := h.Client.ListCollaborators(r.Context(), token, owner, repo)
+	if err != nil {
+		slog.Error("failed to get collaborators", "error", err)
+		collaborators = nil
+	}
+
 	// Render markdown body if present.
 	var renderedBody template.HTML
 	if issue.Body != "" {
@@ -453,6 +460,7 @@ func (h *Handler) IssueDetail(w http.ResponseWriter, r *http.Request) {
 		RenderedBody    template.HTML
 		Comments        []giteaclient.Comment
 		AvailableLabels []giteaclient.Label
+		Collaborators   []string
 	}
 
 	data := templateData{
@@ -460,6 +468,7 @@ func (h *Handler) IssueDetail(w http.ResponseWriter, r *http.Request) {
 		RenderedBody:    renderedBody,
 		Comments:        comments,
 		AvailableLabels: labels,
+		Collaborators:   collaborators,
 	}
 
 	var buf strings.Builder
@@ -660,6 +669,45 @@ func (h *Handler) ApplyLabels(w http.ResponseWriter, r *http.Request) {
 	http.Redirect(w, r, fmt.Sprintf("/issues/%s/%s/%d", owner, repo, index), http.StatusSeeOther)
 }
 
+// AssignIssue handles POST /issues/{owner}/{repo}/{index}/assignees.
+func (h *Handler) AssignIssue(w http.ResponseWriter, r *http.Request) {
+	token := getToken(r)
+	owner := r.PathValue("owner")
+	repo := r.PathValue("repo")
+	indexStr := r.PathValue("index")
+
+	index, err := strconv.ParseInt(indexStr, 10, 64)
+	if err != nil {
+		http.Error(w, "invalid issue index", http.StatusBadRequest)
+		return
+	}
+
+	if err := r.ParseForm(); err != nil {
+		http.Error(w, "bad request", http.StatusBadRequest)
+		return
+	}
+
+	assignee := r.FormValue("assignee")
+	if assignee == "" {
+		http.Error(w, "assignee is required", http.StatusBadRequest)
+		return
+	}
+
+	if err := h.Client.AssignIssue(r.Context(), token, owner, repo, index, []string{assignee}); err != nil {
+		slog.Error("failed to assign issue", "error", err, "owner", owner, "repo", repo, "index", index, "assignee", assignee)
+		http.Error(w, "failed to assign issue", http.StatusInternalServerError)
+		return
+	}
+
+	if isHTMX(r) {
+		w.Header().Set("Content-Type", "text/html; charset=utf-8")
+		fmt.Fprintf(w, `<span style="color:#3fb950">Assigned to %s</span>`, template.HTMLEscapeString(assignee))
+		return
+	}
+
+	http.Redirect(w, r, fmt.Sprintf("/issues/%s/%s/%d", owner, repo, index), http.StatusSeeOther)
+}
+
 // CloseIssue handles POST /issues/{owner}/{repo}/{index}/close.
 func (h *Handler) CloseIssue(w http.ResponseWriter, r *http.Request) {
 	token := getToken(r)