fix: eliminate data races and reduce lock scope

[bosiakov]

• Replace MetadataCache clear() with map reassignment (copy-on-write)
• Snapshot MetadataCache and CurrentQuery under RLock in command list
• Deliver ProcessHotkeyUpdate/ProcessAutostartUpdate results via buffered channels instead of writing UI-thread state from goroutines
• Use closure-captured commands snapshot in click handler instead of re-reading DisplayCommands under RLock
• Release RLock before linear scans in navigateTreeWithSearch and syncTreeToCommandSelection (copy-on-write makes holding unnecessary)
• Replace inline height cache resize with resizeHeightCache call