feat(tui): add vim-style navigation (gg, G, ctrl-u, ctrl-d)

All scrollable panels now support gg (top), G (bottom), ctrl-u (half
page up), and ctrl-d (half page down). Works across dashboard panels
(monitors, detail, maint, logs), fullscreen logs, fullscreen detail,
and settings. Uses a pendingG flag for the two-key gg sequence.

internal/tui/tui.go

@@ -218,6 +218,8 @@ type Model struct {
 	filterMode bool
 	filterText string
 
+	pendingG bool
+
 	// demoMode renders a stable status dot instead of the animated pulse so
 	// screenshots/recordings don't capture the spinner mid-frame. Set via UPTOP_DEMO=1.
 	demoMode bool