feat: add HTTP handlers and health endpoint

Implement all HTTP handlers using Go 1.22+ stdlib ServeMux with
HTMX fragment vs full-page response detection.

- internal/handlers/handlers.go: all route handlers
  - GET /health returns 200 for K8s probes
  - GET / dashboard with triage queue from aggregation layer
  - GET /issues lists all issues across orgs
  - GET /pulls lists all PRs across orgs
  - POST /issues creates issue via aggregation layer
  - POST /issues/{owner}/{repo}/{index}/labels assigns labels
  - POST /pulls/{owner}/{repo}/{index}/review submits PR review
  - HX-Request header detection for HTMX fragment vs full page
  - Mobile-first dark theme base layout with bottom navigation
- cmd/server/main.go: refactored to use centralized route registration
- internal/handlers/handlers_test.go: unit tests for health, dashboard,
  HTMX detection, input validation

Closes leeworks-agents/gitea-mobile#4

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

cmd/server/main.go

@@ -1,14 +1,13 @@
 package main
 
 import (
-	"fmt"
 	"log"
 	"log/slog"
 	"net/http"
 	"os"
-	"strings"
 
 	"gitea.leeworks.dev/0xwheatyz/gitea-mobile/internal/config"
+	giteaclient "gitea.leeworks.dev/0xwheatyz/gitea-mobile/internal/gitea"
 	"gitea.leeworks.dev/0xwheatyz/gitea-mobile/internal/handlers"
 	"gitea.leeworks.dev/0xwheatyz/gitea-mobile/internal/middleware"
 )
@@ -24,33 +23,13 @@ func main() {
 		log.Fatalf("configuration error: %v", err)
 	}
 
-	// Determine if cookies should be marked Secure (disable for local dev).
-	secureCookies := !strings.HasPrefix(cfg.ListenAddr, "localhost") &&
-		!strings.HasPrefix(cfg.ListenAddr, "127.0.0.1")
+	// Create Gitea API client.
+	client := giteaclient.NewClient(cfg.GiteaURL)
 
+	// Create handler with all routes.
 	mux := http.NewServeMux()
-
-	// Health endpoint (no auth required).
-	mux.HandleFunc("GET /health", func(w http.ResponseWriter, r *http.Request) {
-		w.WriteHeader(http.StatusOK)
-		fmt.Fprintln(w, "ok")
-	})
-
-	// Settings handler.
-	settingsHandler := &handlers.SettingsHandler{
-		SessionSecret: cfg.SessionSecret,
-		SecureCookies: secureCookies,
-	}
-	mux.HandleFunc("/settings", settingsHandler.ServeHTTP)
-
-	// Static file server.
-	mux.Handle("/static/", http.StripPrefix("/static/", http.FileServer(http.Dir("static"))))
-
-	// Placeholder dashboard (will be replaced in issue #4/#5).
-	mux.HandleFunc("GET /", func(w http.ResponseWriter, r *http.Request) {
-		w.Header().Set("Content-Type", "text/html; charset=utf-8")
-		fmt.Fprintln(w, "<h1>Gitea Mobile</h1><p>Dashboard coming soon.</p>")
-	})
+	h := handlers.NewHandler(cfg, client)
+	h.RegisterRoutes(mux)
 
 	// Apply middleware chain: logging -> auth.
 	var handler http.Handler = mux