feat(sit): 15s Morse alert tone on sit threshold

The UN notification ding is one short beep — easy to miss in a
meeting. For sit alerts, repeat the system Morse tone (0.7s
staccato "S O S") at 1Hz for 15 ticks, layered on top of the
notification + dock bounce.

- new: SoundPlayer.swift — NSSound-based repeating tone with stop()
- WorkerStore.tickFire: trigger SoundPlayer.playMorseSitAlert(count: 15)
  alongside the existing notify() call
- WorkerStore.sitStop: silence in-flight alert when user stops monitoring

Uses /System/Library/Sounds/Morse.aiff — shipped on every macOS,
no resource bundling needed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Sources/engine/SoundPlayer.swift

@@ -0,0 +1,80 @@
+//
+//  SoundPlayer.swift
+//  摸鱼侠 plugin v0.2.1
+//
+//  Sit-reminder alert tone. The bundled UN notification sound is a
+//  ~1s ding that's easy to miss in a meeting — for sit alerts we want
+//  a 15s nag that makes you actually stand up.
+//
+//  Implementation: repeat the macOS system "Morse" sound (0.7s) once
+//  per second for 15 ticks. Morse is the most distinctive notification
+//  sound shipped with macOS — staccato 3-tone "S O S" pattern that
+//  pierces background noise better than Glass/Hero/Ping/Funk/etc.
+//
+//  Plays via NSSound (not UNNotificationContent.sound) because UN
+//  sounds are limited to ~30s but get truncated to ~5s on macOS, and
+//  attach a single tone to a single notification — no way to fire a
+//  repeated sequence from UN.
+//
+
+import AppKit
+import Foundation
+
+@MainActor
+final class SoundPlayer {
+    static let shared = SoundPlayer()
+    private init() {}
+
+    /// System Morse sound — 0.7s staccato 3-tone "S".
+    /// Shipped on every macOS since at least 10.0; safe to hard-code path.
+    private let morseURL = URL(fileURLWithPath: "/System/Library/Sounds/Morse.aiff")
+
+    private var timer: Timer?
+    private var remaining: Int = 0
+    private var currentSound: NSSound?
+
+    /// Repeated Morse alert: play `count` times at 1Hz. Each play is a
+    /// fresh NSSound so previous-in-flight playback doesn't get stomped
+    /// (NSSound.play() is non-blocking and the 0.7s tone overlaps with
+    /// the next-second silence — acceptable acoustic gap).
+    ///
+    /// Calling while a previous alert is still running cancels the old
+    /// one and restarts from `count`. This keeps "user sat back down +
+    /// trigger fired again 45min later" clean.
+    func playMorseSitAlert(count: Int = 15) {
+        stop()
+        remaining = count
+        playOnce()  // fire immediately, then 1Hz repeat
+        timer = Timer.scheduledTimer(withTimeInterval: 1.0, repeats: true) { [weak self] _ in
+            Task { @MainActor in self?.playOnce() }
+        }
+    }
+
+    /// Cancel any in-flight alert. Called by WorkerStore.sitStop() so
+    /// "user stopped sit monitoring" silences the nag immediately.
+    func stop() {
+        timer?.invalidate()
+        timer = nil
+        remaining = 0
+        currentSound?.stop()
+        currentSound = nil
+    }
+
+    private func playOnce() {
+        guard remaining > 0 else {
+            stop()
+            return
+        }
+        remaining -= 1
+        // byReference: true — load lazily from the system path each call.
+        // The cost is negligible (< 1ms) and avoids keeping a 200KB AIFF
+        // resident across the 45min idle stretch between alerts.
+        if let sound = NSSound(contentsOf: morseURL, byReference: true) {
+            sound.play()
+            currentSound = sound
+        } else {
+            WorkerDebugLog.write("SoundPlayer: failed to load \(morseURL.path)")
+            stop()
+        }
+    }
+}

Sources/engine/WorkerStore.swift

@@ -256,7 +256,11 @@ final class WorkerStore: ObservableObject {
                         title: "该起来动一下了",
                         body: "你已连续坐了 \(sitTriggerMin) 分钟，起身喝口水吧。"
                     )
-                    WorkerDebugLog.write("sit threshold notification fired (\(sitElapsedSec)s)")
+                    // 1Hz × 15 ticks of system Morse tone — the UN
+                    // notification ding is too easy to miss in a meeting,
+                    // so the sit alert gets a louder, longer signal.
+                    SoundPlayer.shared.playMorseSitAlert(count: 15)
+                    WorkerDebugLog.write("sit threshold notification + morse alert fired (\(sitElapsedSec)s)")
                 }
             }
 
@@ -463,6 +467,9 @@ final class WorkerStore: ObservableObject {
         defaults.set(false, forKey: K.sitEnabled)
         defaults.set(0, forKey: K.sitAccumActive)
         defaults.set(0.0, forKey: K.sitLastNotified)
+        // Silence any in-flight Morse alert from a prior trigger so the
+        // user stopping monitoring also stops the nag immediately.
+        SoundPlayer.shared.stop()
     }
 
     func sitSetTrigger(_ min: Int) {