🎨 Palette: Fix IconButton touch targets

[yuga-hashimoto]

💡 What
Removed Modifier.size(24.dp) from several IconButton components across MainActivity.kt and ChatActivity.kt.

🎯 Why
Android and Material Design guidelines stipulate a minimum touch target size of 48x48dp for interactive elements to ensure accessibility for users with varying dexterity levels. Explicitly constraining the size of an IconButton (which internally provides padding to meet this target) to 24dp overrides this behavior, making the buttons harder to tap.

♿ Accessibility
By removing the explicit size constraint, the IconButton components naturally expand to provide the standard, accessible 48x48dp minimum touch target area without altering the visual size of the inner Icon (which remains appropriately scaled). This significantly improves the UX for all users, particularly those relying on accessible touch targets.

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PR created automatically by Jules for task 3506056329231000616 started by @yuga-hashimoto

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