Add server health endpoint and status page

Backend:
- New /api/server/health endpoint returning server, paths, disk,
  mods, steamcmd, and frontend status
- Health endpoint checks directory existence/writability, disk usage,
  server binary, and frontend dist availability

Frontend:
- New Status page (route /status) with sections for Server, SteamCMD,
  Mods, Frontend, Directories (with exists/writable badges), and
  Disk Usage (with size formatting and used-percent coloring)
- Paths are clickable to copy and use start-truncation (ellipsis on
  the left) with native hover tooltip
- Health API client integration with 10s auto-refresh

Misc:
- Update .gitignore and Makefile, minor Layout nav addition

backend/internal/api/health.go

@@ -0,0 +1,188 @@
+package api
+
+import (
+	"os"
+	"path/filepath"
+	"runtime"
+	"syscall"
+
+	"arma3-web-server/internal/services"
+
+	"github.com/gin-gonic/gin"
+	"golang.org/x/sys/unix"
+)
+
+type HealthResponse struct {
+	Server   ServerHealth   `json:"server"`
+	Paths    []PathHealth   `json:"paths"`
+	Mods     ModsHealth     `json:"mods"`
+	Disk     []DiskHealth   `json:"disk"`
+	SteamCMD SteamCMDHealth `json:"steamcmd"`
+	Frontend FrontendHealth `json:"frontend"`
+}
+
+type ServerHealth struct {
+	Running    bool   `json:"running"`
+	Installed  bool   `json:"installed"`
+	BinaryPath string `json:"binary_path,omitempty"`
+}
+
+type PathHealth struct {
+	Path     string `json:"path"`
+	Exists   bool   `json:"exists"`
+	Writable bool   `json:"writable"`
+}
+
+type ModsHealth struct {
+	WorkshopCount int `json:"workshop_count"`
+	LocalCount    int `json:"local_count"`
+}
+
+type DiskHealth struct {
+	Path        string  `json:"path"`
+	TotalBytes  uint64  `json:"total_bytes"`
+	FreeBytes   uint64  `json:"free_bytes"`
+	UsedPercent float64 `json:"used_percent"`
+}
+
+type SteamCMDHealth struct {
+	Running bool `json:"running"`
+}
+
+type FrontendHealth struct {
+	Served bool   `json:"served"`
+	Path   string `json:"path"`
+}
+
+func (h *Handler) Health(c *gin.Context) {
+	workshopMods := services.ListWorkshopMods(h.serverfileDir)
+	localMods := services.ListLocalMods(h.modsDir)
+
+	paths := h.checkPaths()
+	disk := h.checkDisk()
+	frontendServed, _ := h.checkFrontend()
+
+	resp := HealthResponse{
+		Server: ServerHealth{
+			Running:    h.process.IsRunning(),
+			Installed:  checkServerBinary(h.serverfileDir),
+			BinaryPath: serverBinaryPath(h.serverfileDir),
+		},
+		Paths: paths,
+		Mods: ModsHealth{
+			WorkshopCount: len(workshopMods),
+			LocalCount:    len(localMods),
+		},
+		Disk: disk,
+		SteamCMD: SteamCMDHealth{
+			Running: h.steamcmd.IsRunning(),
+		},
+		Frontend: FrontendHealth{
+			Served: frontendServed,
+			Path:   h.frontendDir,
+		},
+	}
+
+	c.JSON(200, resp)
+}
+
+func (h *Handler) checkPaths() []PathHealth {
+	dirs := []string{
+		h.dataDir,
+		filepath.Join(h.dataDir, "modlists"),
+		h.serverfileDir,
+		h.modsDir,
+		h.cfgDir,
+		h.profilesDir,
+	}
+
+	result := make([]PathHealth, 0, len(dirs))
+	for _, dir := range dirs {
+		p := PathHealth{Path: dir}
+		fi, err := os.Stat(dir)
+		if err == nil && fi.IsDir() {
+			p.Exists = true
+			p.Writable = isWritable(dir)
+		}
+		result = append(result, p)
+	}
+	return result
+}
+
+func (h *Handler) checkDisk() []DiskHealth {
+	paths := []string{h.dataDir, h.serverfileDir}
+	seen := make(map[string]bool)
+	var result []DiskHealth
+
+	for _, p := range paths {
+		info, err := diskUsage(p)
+		if err != nil {
+			continue
+		}
+		key := info.Path
+		if seen[key] {
+			continue
+		}
+		seen[key] = true
+		result = append(result, info)
+	}
+	return result
+}
+
+func (h *Handler) checkFrontend() (bool, string) {
+	if h.frontendDir == "" {
+		return false, ""
+	}
+	fi, err := os.Stat(h.frontendDir)
+	return err == nil && fi.IsDir(), h.frontendDir
+}
+
+func checkServerBinary(serverfileDir string) bool {
+	_, found := findServerBinary(serverfileDir)
+	return found
+}
+
+func serverBinaryPath(serverfileDir string) string {
+	path, _ := findServerBinary(serverfileDir)
+	return path
+}
+
+func findServerBinary(serverfileDir string) (string, bool) {
+	candidates := []string{"arma3server_x64"}
+	if runtime.GOOS == "windows" {
+		candidates = append(candidates, "arma3server_x64.exe")
+	} else {
+		candidates = append(candidates, "arma3server_x64.exe")
+	}
+	for _, name := range candidates {
+		p := filepath.Join(serverfileDir, name)
+		fi, err := os.Stat(p)
+		if err == nil && !fi.IsDir() {
+			return p, true
+		}
+	}
+	return "", false
+}
+
+func isWritable(path string) bool {
+	return unix.Access(path, unix.W_OK) == nil
+}
+
+func diskUsage(path string) (DiskHealth, error) {
+	var stat syscall.Statfs_t
+	if err := syscall.Statfs(path, &stat); err != nil {
+		return DiskHealth{}, err
+	}
+	total := stat.Blocks * uint64(stat.Bsize)
+	free := stat.Bfree * uint64(stat.Bsize)
+	var usedPercent float64
+	if total > 0 {
+		usedPercent = float64(total-free) / float64(total) * 100
+	}
+	return DiskHealth{
+		Path:        path,
+		TotalBytes:  total,
+		FreeBytes:   free,
+		UsedPercent: usedPercent,
+	}, nil
+}