fix(persist): flush pending save on navigate-away and quit

Edits are saved on an 800ms debounce, so leaving a book within that
window lost the latest changes. Flush a dirty book before openBook /
closeBook swap state (covers All books, open-another, Cmd+N), and add a
window close-request handler that saves before destroying on quit.

Claude-Session: https://claude.ai/code/session_01RzTWrLiy4zGuw2hZSQGamk

src-tauri/capabilities/default.json

@@ -5,6 +5,7 @@
   "windows": ["main"],
   "permissions": [
     "core:default",
+    "core:window:allow-destroy",
     "opener:default",
     "dialog:default",
     "updater:default",