fix(usage): refresh usage snapshot on window focus

[posthog]

Problem

• The title-bar usage badge shows materially lower usage than the website for the same account, even after the website reports the limit as already reached.
• UsageMonitorService serves the badge from a cached snapshot, refreshed only by two triggers: this app's own LLM activity, or a 30-minute backstop timer.
• Usage consumed elsewhere — the website, another device, a teammate on the same org — never reaches the desktop app's cache until that backstop fires, so a user who checks the badge after leaving the window unfocused for a while can see a stale, lower number than reality.

Changes

• Adds a window-focus trigger to UsageMonitorService, mirroring the existing onLlmActivity pattern: UsageHost gains onWindowFocus/offWindowFocus, and the service calls requestRefresh() whenever the window regains focus.
• Wires it in the desktop main-process container via the existing IMainWindow.onFocus platform hook — no new Electron surface needed.

How did you test this?

• pnpm --filter @posthog/core test -- usage-monitor — all 21 tests pass, including 2 new ones covering the focus-triggered refresh and its unsubscribe-on-stop behavior.
• npx biome check on all changed files — clean.
• Confirmed the pre-existing tsc --noEmit failures for @posthog/agent/@posthog/git reproduce identically on a clean main checkout (unbuilt workspace deps in this sandbox), unrelated to this change; no new type errors were introduced.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit f8ce1f4.}