Add Manual Refresh Button

[Technical-1]

Priority: Low
Labels: ui, enhancement

Description:
Add refresh button in top panel. Allow manual refresh independent of auto-refresh. Show last refresh timestamp.

Files to modify:

• src/main.rs

Acceptance criteria:

• Refresh button added to top panel
• Manual refresh triggers immediate update
• Last refresh timestamp displayed
• Button provides visual feedback when clicked
• Works independently of auto-refresh

Implementation notes:

• Add egui::Button labeled "Refresh" in top panel
• Store last_refresh: Instant in RustDashboardApp
• On button click, trigger immediate refresh
• Display timestamp formatted as "Last refresh: HH:MM:SS"
• Consider disabling button during refresh to prevent spam