feat: implement issue and PR detail handlers with routes

Add GET /issues/{owner}/{repo}/{index} and GET /pulls/{owner}/{repo}/{index}
routes that fetch individual issues/PRs from the Gitea API and render them
using the existing issue_detail.html and pull_detail.html templates.

New client methods:
- GetIssue: fetch a single issue by owner/repo/index
- GetPull: fetch a single pull request by owner/repo/index
- GetIssueComments: fetch comments for an issue
- GetRepoLabels: fetch available labels for a repository

Both handlers support HTMX fragment responses and full-page rendering,
consistent with the existing handler pattern.

Closes leeworks-agents/gitea-mobile#24

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

internal/handlers/handlers.go

@@ -7,6 +7,7 @@ import (
 	"log/slog"
 	"net/http"
 	"strconv"
+	"strings"
 
 	"gitea.leeworks.dev/0xwheatyz/gitea-mobile/internal/config"
 	giteaclient "gitea.leeworks.dev/0xwheatyz/gitea-mobile/internal/gitea"
@@ -40,8 +41,12 @@ func (h *Handler) RegisterRoutes(mux *http.ServeMux) {
 	mux.HandleFunc("POST /issues", h.CreateIssue)
 	mux.HandleFunc("POST /issues/{owner}/{repo}/{index}/labels", h.ApplyLabels)
 
+	// Issue detail.
+	mux.HandleFunc("GET /issues/{owner}/{repo}/{index}", h.IssueDetail)
+
 	// Pull requests.
 	mux.HandleFunc("GET /pulls", h.ListPulls)
+	mux.HandleFunc("GET /pulls/{owner}/{repo}/{index}", h.PullDetail)
 	mux.HandleFunc("POST /pulls/{owner}/{repo}/{index}/review", h.SubmitReview)
 
 	// Settings (handled separately for auth bypass).
@@ -329,6 +334,116 @@ func (h *Handler) ListPulls(w http.ResponseWriter, r *http.Request) {
 	renderPage(w, r, "Pull Requests", "pulls", content)
 }
 
+// IssueDetail handles GET /issues/{owner}/{repo}/{index}.
+func (h *Handler) IssueDetail(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
+	}
+
+	issue, err := h.Client.GetIssue(r.Context(), token, owner, repo, index)
+	if err != nil {
+		slog.Error("failed to get issue", "error", err, "owner", owner, "repo", repo, "index", index)
+		renderPage(w, r, "Issue Not Found", "issues",
+			`<h1>Issue Not Found</h1><p class="empty">Could not load the requested issue.</p>`)
+		return
+	}
+
+	comments, err := h.Client.GetIssueComments(r.Context(), token, owner, repo, index)
+	if err != nil {
+		slog.Error("failed to get comments", "error", err)
+		comments = nil // non-fatal, render without comments
+	}
+
+	labels, err := h.Client.GetRepoLabels(r.Context(), token, owner, repo)
+	if err != nil {
+		slog.Error("failed to get repo labels", "error", err)
+		labels = nil
+	}
+
+	// Build the content HTML using the template.
+	tmpl, err := template.ParseFiles("internal/templates/issue_detail.html")
+	if err != nil {
+		slog.Error("failed to parse issue_detail template", "error", err)
+		http.Error(w, "template error", http.StatusInternalServerError)
+		return
+	}
+
+	type templateData struct {
+		Issue           *giteaclient.Issue
+		Comments        []giteaclient.Comment
+		AvailableLabels []giteaclient.Label
+	}
+
+	data := templateData{
+		Issue:           issue,
+		Comments:        comments,
+		AvailableLabels: labels,
+	}
+
+	var buf strings.Builder
+	if err := tmpl.ExecuteTemplate(&buf, "content", data); err != nil {
+		slog.Error("failed to execute issue_detail template", "error", err)
+		http.Error(w, "template error", http.StatusInternalServerError)
+		return
+	}
+
+	renderPage(w, r, fmt.Sprintf("Issue #%d", index), "issues", buf.String())
+}
+
+// PullDetail handles GET /pulls/{owner}/{repo}/{index}.
+func (h *Handler) PullDetail(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 PR index", http.StatusBadRequest)
+		return
+	}
+
+	pr, err := h.Client.GetPull(r.Context(), token, owner, repo, index)
+	if err != nil {
+		slog.Error("failed to get pull request", "error", err, "owner", owner, "repo", repo, "index", index)
+		renderPage(w, r, "PR Not Found", "pulls",
+			`<h1>Pull Request Not Found</h1><p class="empty">Could not load the requested pull request.</p>`)
+		return
+	}
+
+	// Build the content HTML using the template.
+	tmpl, err := template.ParseFiles("internal/templates/pull_detail.html")
+	if err != nil {
+		slog.Error("failed to parse pull_detail template", "error", err)
+		http.Error(w, "template error", http.StatusInternalServerError)
+		return
+	}
+
+	type templateData struct {
+		Pull *giteaclient.PullRequest
+	}
+
+	data := templateData{
+		Pull: pr,
+	}
+
+	var buf strings.Builder
+	if err := tmpl.ExecuteTemplate(&buf, "content", data); err != nil {
+		slog.Error("failed to execute pull_detail template", "error", err)
+		http.Error(w, "template error", http.StatusInternalServerError)
+		return
+	}
+
+	renderPage(w, r, fmt.Sprintf("PR #%d", index), "pulls", buf.String())
+}
+
 // CreateIssue handles POST /issues.
 func (h *Handler) CreateIssue(w http.ResponseWriter, r *http.Request) {
 	token := getToken(r)