feat: add proper feature flag system (replace env var hacks)

[ryanmagoon]

Problem

We're using GAMELORD_HW_SAVE_STATES=1 as an env var to toggle experimental features, which is clunky:

• Utility processes don't inherit shell env vars (had to plumb it through init commands)
• No UI for toggling — requires restarting the app
• No persistence — have to set it every time
• Doesn't scale as we add more experimental features

Proposal

Add a proper feature flag system. Options to evaluate:

• Vercel Flags SDK — designed for this, integrates with the Vercel ecosystem
• Simple local flags — JSON file in userData with a Settings > Experimental panel to toggle flags
• LaunchDarkly / Statsig — overkill for a desktop app but worth noting

Current flags that would benefit

• hwSaveStates — Enable save states for HW-render cores (Dolphin). Currently crashes due to upstream bug, being fixed in parallel (fix: Dolphin save states crash the emulation worker (segfault in retro_serialize) #342)
• Future: shader debug mode, frame timing overlay, experimental cores, etc.

Requirements

• Flags accessible from main process, worker processes, and renderer
• Persistent across sessions
• Toggleable from the UI (Settings > Experimental or similar)
• No restart required (ideally)