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{}{

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)

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">