feat(main): prevent multiple app instances with single-instance lock

[rxm96]

What

Adds an Electron single-instance lock so only one DropPilot process can run at a time. A second launch surfaces the already-running window instead of starting a duplicate process.

Why

DropPilot defaults to close-to-tray, so the window is usually hidden-but-alive rather than closed. Without a guard, double-clicking the exe (or auto-start firing while a manual instance is already running) spins up a second process — two trackers, two PubSub connections, two trays competing over the same Twitch session.

How

In src/main/index.ts:

• Call app.requestSingleInstanceLock() before startup. The losing instance calls app.quit() and never creates a window or starts Twitch/IPC services.
• The winning instance handles the second-instance event by restoring the existing window from the tray (restore()/show()) and focusing it — chosen behavior: bring the running window to front.
• A belt-and-suspenders early-return at the top of the app.whenReady() callback (if (!gotSingleInstanceLock) return;) guarantees a losing instance does no startup work even if ready still fires.

Verification

• npm run typecheck (both tsconfigs): clean
• npm run format:check / prettier: clean
• npm run lint: clean for the changed file (pre-existing warnings only, in unrelated renderer/.../watch hooks)
• npm test: 514 passed

index.ts is main-process lifecycle wiring and isn't unit-testable (consistent with the project's pure-function-only test convention), so there's no automated test for this path.

Reviewer notes

• Not yet runtime-verified end-to-end (launching the built app twice and observing the window surface). The logic is the standard documented Electron single-instance pattern.
• Applied unconditionally, including dev. npm run dev only ever launches one instance and vite-plugin-electron releases the lock on hot-restart; if it ever interferes, guarding with app.isPackaged is a one-line fallback.
• Auto-update unaffected: quitAndInstall exits the old process (releasing the lock) before the new one launches.

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