feat(settings): add launcher and instance editors

[objz]

summary

this adds launcher and per-instance settings editors directly to the TUI, with $EDITOR only used as a fallback.

settings

• launcher settings for appearance, launch defaults, content, storage, notifications, and maintenance
• instance settings for the game/runtime, Java, memory, window behavior, and commands
• keyboard-driven pickers for Java, accounts, GLFW, themes, providers, and resolutions
• wrapping tag editors for JVM arguments and environment variables
• shortcut guide presets and per-area visibility controls for main panels and popups

behavior

• completed changes save automatically
• runtime changes are confirmed and repaired before they are saved
• launcher defaults can be inherited explicitly by instances
• existing configs receive missing settings without replacing current values, comments, or custom keys
• settings that need a restart are marked in the UI

closes #29
closes #42