Fix empty NavDisplay backstack by removing redundant onBack calls

[mikef-dk]

Starting with version 1.8.0 of Flocon everytime the Panel is closed an IllegalArgumentException is thrown:

java.lang.IllegalArgumentException: NavDisplay backstack cannot be empty
	at androidx.navigation3.ui.NavDisplayKt__NavDisplayKt.NavDisplay(NavDisplay.kt:359)
	at androidx.navigation3.ui.NavDisplayKt.NavDisplay(Unknown Source)
	at androidx.navigation3.ui.NavDisplayKt__NavDisplayKt.NavDisplay$lambda$4$NavDisplayKt__NavDisplayKt(NavDisplay.kt)
	at androidx.compose.runtime.RecomposeScopeImpl.compose(RecomposeScopeImpl.kt:204)
	at androidx.compose.runtime.GapComposer.recomposeToGroupEnd(GapComposer.kt:1678)
	at androidx.compose.runtime.GapComposer.skipCurrentGroup(GapComposer.kt:2014)
	at androidx.compose.runtime.GapComposer.doCompose-aFTiNEg(GapComposer.kt:2655)
	at androidx.compose.runtime.GapComposer.recompose-aFTiNEg$runtime(GapComposer.kt:2577)

This should be related to the fact that onBack was triggered multiple times. Removing it from the onClick callbacks fixes this issue.

[gemini-code-assist]

Code Review

This pull request removes redundant onBack() calls in PanelScene.kt to prevent double-navigation when hiding a panel. The reviewer noted a similar issue in the EscapeHandler of FloconPanel.kt that should be addressed to prevent potential crashes when closing the panel with the Escape key.