feat: improve daemon resilience, streaming, and usenet downloads

- Add daemon state persistence and stale resume file cleanup
- Add TriggerPoll for WebSocket resume actions
- Improve stream server with graceful shutdown and connection tracking
- Add desktop notifications for download completion
- Add media file organization with Movies/TV Shows detection
- Improve usenet downloader with progress tracking and resume support
- Add self-update package with GitHub release verification
- Downgrade tablewriter to v0.0.5 (v1.x API breaking change)

internal/agent/state.go

@@ -0,0 +1,72 @@
+package agent
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"time"
+
+	"github.com/torrentclaw/torrentclaw-cli/internal/config"
+)
+
+// DaemonState is written to disk every heartbeat for external tools to read.
+type DaemonState struct {
+	AgentID         string         `json:"agentId"`
+	Status          string         `json:"status"` // running | upgrading | shutting_down
+	Version         string         `json:"version"`
+	PID             int            `json:"pid"`
+	StartedAt       time.Time      `json:"startedAt"`
+	LastHeartbeat   time.Time      `json:"lastHeartbeat"`
+	ActiveTasks     int            `json:"activeTasks"`
+	CompletedCount  int            `json:"completedCount"`
+	FailedCount     int            `json:"failedCount"`
+	TotalDownloaded int64          `json:"totalDownloaded"`
+	MethodStats     map[string]int `json:"methodStats,omitempty"`
+}
+
+// stateFilePathFn is overridable for testing.
+var stateFilePathFn = func() string {
+	return filepath.Join(config.DataDir(), "daemon.state.json")
+}
+
+// StateFilePath returns the path to the daemon state file.
+func StateFilePath() string {
+	return stateFilePathFn()
+}
+
+// WriteState writes the daemon state to disk (best-effort, never errors).
+func WriteState(state *DaemonState) {
+	path := StateFilePath()
+	dir := filepath.Dir(path)
+	os.MkdirAll(dir, 0o755)
+
+	data, err := json.MarshalIndent(state, "", "  ")
+	if err != nil {
+		return
+	}
+
+	// Write to temp file then rename for atomicity
+	tmp := path + ".tmp"
+	if err := os.WriteFile(tmp, data, 0o644); err != nil {
+		return
+	}
+	os.Rename(tmp, path)
+}
+
+// ReadState reads the daemon state from disk. Returns nil if not found.
+func ReadState() *DaemonState {
+	data, err := os.ReadFile(StateFilePath())
+	if err != nil {
+		return nil
+	}
+	var state DaemonState
+	if json.Unmarshal(data, &state) != nil {
+		return nil
+	}
+	return &state
+}
+
+// RemoveState deletes the state file (called on clean shutdown).
+func RemoveState() {
+	os.Remove(StateFilePath())
+}