fix: disable Hold mode on Linux when /dev/input permissions are missing

[xAlcahest]

Summary

On Linux, Hold mode (push-to-talk) requires read access to /dev/input/event* devices via the input group. Previously, Hold mode was always selectable even when the user lacked permissions, resulting in a broken hotkey with no feedback. Now the app proactively checks /dev/input permissions at startup and disables the Hold button when access is missing, showing setup instructions inline instead of waiting for the error to happen.

Changes

• src/helpers/linuxKeyManager.js added hasInputPermissions() that checks if any /dev/input/event* file is readable by the process
• src/helpers/ipcHandlers.js supportsPushToTalk now requires both binary availability and input device permissions
• src/components/ui/ActivationModeSelector.tsx added pushDisabled prop to grey out the Hold button when unsupported
• src/components/SettingsPage.tsx proactive permission check on mount, auto-revert to Tap on permission-denied, LinuxPttSetupInfo shown when Hold is disabled
• src/components/OnboardingFlow.tsx same proactive check and disabled Hold button during onboarding
• main.js auto-reverts activation mode to Tap on permission-denied to prevent the hotkey from staying broken

Fixes #611