feat: Add macOS widget support

[validatedev]

TL;DR

Adds native macOS WidgetKit support for OpenUsage. Each widget instance can show one configured provider in small, medium, or large sizes, backed by display-only snapshots exported by the host app.

What was happening

• OpenUsage usage data was only available from the menu-bar popover.
• The SwiftPM host had no WidgetKit extension, shared data bridge, or widget deep-link routing.
• Local and release build scripts did not embed or sign an app extension.

What this changes

• Adds a WidgetKit extension for Claude, Codex, Cursor, Antigravity, Copilot, Devin, Grok, OpenRouter, and Z.ai.
• Supports small, medium, and large widgets with provider configuration, stale/error states, reset dates, and metric ordering inherited from OpenUsage.
• Publishes privacy-safe, display-ready snapshots through a versioned App Group bridge; the extension never reads credentials or contacts provider APIs.
• Opens the relevant dashboard provider from a widget, or routes disabled/empty providers to Customize.
• Adds extension embedding, Team ID extraction, nested signing, universal release builds, and release validation to the existing scripts.
• Documents widget behavior, refresh timing, debugging, architecture, and the provider-registration checklist.
• Uses a 30-minute stale threshold to account for WidgetKit's background update budget.

Heads-up

• A valid Apple Development identity and Apple WWDR certificate are required for local WidgetKit embedding. Without one, the normal development script still builds the host app and warns that widget support was omitted.
• Screenshots are intentionally pending while this PR remains a draft; add them before marking it ready for review.

Tests

• swift test: 900 XCTest cases passed, 3 skipped; 3 Swift Testing cases also passed.
• xcodebuild -project OpenUsageWidgets.xcodeproj -scheme OpenUsageWidgets -configuration Debug CODE_SIGNING_ALLOWED=NO build: passed.
• ./script/build_and_run.sh: built and launched a team-signed host with the signed widget extension embedded.
• codesign --verify --deep --strict dist/OpenUsage.app: passed.
• git diff --check: passed.

Screenshots

Pending before this draft is marked ready for review.

[validatedev]

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[chatgpt-codex-connector]

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