refactor(main): dedupe window restore+focus into a shared focusWindow helper

[PaulHorn]

What

Extracts a small focusWindow(win) helper in src/main/singleInstance.ts and routes the three places that surface an existing window through it.

Why

The un-minimize-then-focus idiom —

if (win.isMinimized()) win.restore()
win.focus()

— was duplicated across three call sites:

• singleInstance.focusRunningInstanceWindow (second-instance event)
• deliverOpenPath in index.ts (open-path)
• the notification-click handler in ipc/notifications.ts

PR #224 introduced the tested focusRunningInstanceWindow helper for the single-instance case but left the two pre-existing inline copies in place. This consolidates all three so there's a single place that decides how the app brings an existing window to the foreground.

How

• New focusWindow(win: BrowserWindow) in singleInstance.ts.
• focusRunningInstanceWindow now picks the first live window and delegates to focusWindow.
• deliverOpenPath and the notification handler call focusWindow (each keeps its own surrounding guard — try/catch and !isDestroyed() respectively).

Notes

• No behavior change. Pure de-duplication.
• Each call site keeps its existing window selection logic (findMainWindow, the notification's owner window, first-live); only the shared restore+focus tail is unified.

Tests

• Adds direct unit tests for focusWindow (focuses; restores-then-focuses when minimized).
• tsc --noEmit clean · full vitest suite green (529 incl. 8 in singleInstance.test.ts).