docs: backfill missing Swift doc comments on public APIs

[tannn]

Summary

• Adds type-level and member-level Swift doc comments to seven source files that had gaps in public API documentation
• No behaviour changes; all edits are additive doc comment only

Files changed

• FloatingPanelComponents.swift — type-level docs on FloatingPanelDesignTokens, FloatingPanelLoadingView, FloatingPanelResponseCard, FloatingPanelErrorCallout; parameter docs on each public init
• FloatingPanelView.swift — type-level doc on FloatingPanelView
• MenuBarCoordinator.swift — class doc, installStatusItemIfNeeded, uninstallStatusItem, callback property docs
• Logger.swift — Logger enum doc, all Category cases, four static log-level methods
• FloatingPanelViewModel.swift — setErrorMessage(_:) and copyText
• AppState.swift — eleven convenience setter methods
• DiagnosticEvent.swift — four public static redaction helpers

Test plan

• Verify Swift build succeeds with no warnings
• Review doc comments render correctly in Xcode Quick Help

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