perf: add adaptive polling and fix lyrics interval leak

- Playback state polling: 1000ms when paused, 250ms when playing
- Audio thread timeout: 500ms when idle, 100ms when playing
- Lyrics interval: self-check to auto-stop when conditions not met
- Add cleanup return to lyrics  in +page.svelte
- Add build-and-run.sh and monitor-resources.sh scripts
- Add OPTIMIZATION_PLAN.md documentation

scripts/monitor-resources.sh

@@ -0,0 +1,52 @@
+#!/bin/bash
+# Monitor QBZ resource usage
+# Usage: ./scripts/monitor-resources.sh
+
+echo "=== QBZ Resource Monitor ==="
+echo "Waiting for QBZ process..."
+echo ""
+
+# Wait for process to start
+while ! pgrep -f "qbz-nix" > /dev/null 2>&1; do
+    sleep 0.5
+done
+
+PID=$(pgrep -f "qbz-nix" | head -1)
+echo "Found QBZ PID: $PID"
+echo ""
+echo "Press Ctrl+C to stop monitoring"
+echo "==========================================="
+echo ""
+
+# Monitor loop
+while true; do
+    if ! ps -p "$PID" > /dev/null 2>&1; then
+        echo "QBZ process ended"
+        exit 0
+    fi
+
+    # Get stats
+    CPU=$(ps -p "$PID" -o %cpu= 2>/dev/null | tr -d ' ')
+    MEM=$(ps -p "$PID" -o rss= 2>/dev/null | tr -d ' ')
+    MEM_MB=$((MEM / 1024))
+    THREADS=$(ps -p "$PID" -o nlwp= 2>/dev/null | tr -d ' ')
+
+    # Get WebKit process if exists
+    WEBKIT_PID=$(pgrep -f "WebKitWebProcess" | head -1 2>/dev/null)
+    if [[ -n "$WEBKIT_PID" ]]; then
+        WEBKIT_MEM=$(ps -p "$WEBKIT_PID" -o rss= 2>/dev/null | tr -d ' ')
+        WEBKIT_MB=$((WEBKIT_MEM / 1024))
+        WEBKIT_CPU=$(ps -p "$WEBKIT_PID" -o %cpu= 2>/dev/null | tr -d ' ')
+    else
+        WEBKIT_MB=0
+        WEBKIT_CPU=0
+    fi
+
+    TOTAL_MB=$((MEM_MB + WEBKIT_MB))
+
+    # Print stats
+    printf "\r[QBZ] CPU: %5s%% | RAM: %4dMB | Threads: %2s | [WebKit] CPU: %5s%% | RAM: %4dMB | TOTAL: %4dMB" \
+        "$CPU" "$MEM_MB" "$THREADS" "$WEBKIT_CPU" "$WEBKIT_MB" "$TOTAL_MB"
+
+    sleep 1
+done