Add quota warning thresholds

[Alekstodo]

Summary

• Closes Allow configurable alert thresholds with notification bubbles and sounds. #617 by adding opt-in quota warning notifications for session and weekly quota windows.
• Adds sanitized global threshold settings with defaults of 50% and 20% remaining, plus a global notification sound toggle.
• Adds provider/window overrides that live in ProviderConfig and inherit global settings until customized.
• Reuses the existing SessionQuotaNotifier / UsageStore notification path and keeps depleted/restored 0% behavior unchanged.

Notes

• Reviewed the intent behind PR Add configurable quota warning notifications #433, but did not port it directly because it is stale/conflicting and duplicates the current notification architecture.
• This PR intentionally stays scoped to the minimal Allow configurable alert thresholds with notification bubbles and sounds. #617 surface: macOS notification bubbles, sound toggle, configurable thresholds, and provider/window overrides. It does not add a Notifications tab, Shortcuts, or webhooks.

Testing

• DEVELOPER_DIR=/Applications/Xcode.app/Contents/Developer swift test --filter QuotaWarning
• DEVELOPER_DIR=/Applications/Xcode.app/Contents/Developer swift test --filter UsageStoreSessionQuotaTransitionTests
• DEVELOPER_DIR=/Applications/Xcode.app/Contents/Developer swift test --filter SettingsStoreTests
• DEVELOPER_DIR=/Applications/Xcode.app/Contents/Developer swift test --filter CodexActiveSourceConfigTests
• DEVELOPER_DIR=/Applications/Xcode.app/Contents/Developer ./Scripts/lint.sh lint
• DEVELOPER_DIR=/Applications/Xcode.app/Contents/Developer swift test builds and runs, but the full suite currently reports 4 failures in existing HistoricalUsagePace tests unrelated to this change.
• DEVELOPER_DIR=/Applications/Xcode.app/Contents/Developer ./Scripts/compile_and_run.sh builds the release binary, then fails during ad-hoc codesigning/packaging because the generated CodexBar.app / bundled Sparkle.framework has macOS resource-fork/Finder metadata.

[Alekstodo]

Closing for now while we continue local validation before opening the upstream PR.