fix: recover widgets lost during monitor connect/disconnect on macOS

[lohrm-spf]

Summary

Fixes a bug where Zebar's bar silently disappears when connecting/disconnecting external monitors on macOS while the process and tray icon remain alive.

Problem

During macOS display reconfiguration, Tauri's available_monitors() can transiently return an empty or partial list. The existing code path:

1. relaunch_all removes all widget states from the HashMap before attempting recreation
2. widget_coordinates returns an empty Vec when no monitors match → the creation loop runs zero iterations → returns Ok(())
3. Widget state is permanently lost — no error logged, no recovery attempted
4. Additionally, ? operator in the per-widget relaunch loop bails on the first failure, skipping all remaining widgets whose states were already deleted

Result: process alive (tray icon visible), zero widget windows, empty error log.

Fix

Three layered defenses:

• Skip empty monitor broadcasts (monitor_state.rs): When available_monitors() returns an empty list during display transition, skip the update to prevent premature relaunches with zero monitors
• Per-widget error resilience (widget_factory.rs): Collect errors instead of bailing on first failure, so one widget's error doesn't prevent siblings from relaunching. Also adds warn! when no monitors match a widget's selection
• Startup recovery with backoff (main.rs): After a monitor change relaunch, if widget_states is empty but startup configs exist, retry startup() with exponential backoff (500ms/1000ms/1500ms)

Test plan

• Confirm bar disappears on monitor disconnect (before fix)
• Verify bar recovers after monitor disconnect/reconnect (after fix)
• Verify normal startup and manual widget open/close still work
• Verify multi-monitor setup creates widgets on all monitors