Add localization catalog, helpers and en/zh Localizable.strings for app & widget

[waffensam]

Motivation

• Centralize all user-facing strings to avoid continued use of bare literals and to provide a single source of truth for App and Widget text.
• Ensure localized resources are packaged by SwiftPM and that English is kept as the development baseline with zh-Hans provided as a Simplified Chinese translation.
• Make it easy for SwiftUI/AppKit call sites to use typed helpers (LocalizedStringResource/String(localized:)-style access) and to add further keys by domain.

Description

• Add LocalizationCatalog in Sources/CodexBarCore/LocalizationCatalog.swift which defines namespaced keys and English fallbacks (e.g. menu.settings, preferences.general.title, alert.login.codex_missing_cli.title, widget.empty.open_app).
• Add target-specific helpers AppStrings (Sources/CodexBar/AppStrings.swift) and WidgetStrings (Sources/CodexBarWidget/WidgetStrings.swift) that expose string(_:), text(_:), formatted(_:) and resource(_:) convenience methods for app and widget usage.
• Add English and Simplified Chinese Localizable.strings under Sources/CodexBar/Resources/{en,zh-Hans}.lproj and equivalent files under Sources/CodexBarWidget/Resources/, and update Package.swift to set defaultLocalization: "en" and to process widget resources (resources: [.process("Resources")]) so SwiftPM will include them.
• Migrate existing literal strings in Sources/CodexBar/PreferencesDisplayPane.swift, Sources/CodexBar/StatusItemController+Actions.swift (Codex login alerts), and Sources/CodexBarWidget/* (empty states, metrics, and intent titles/descriptions) to the new helpers, and add Tests/CodexBarTests/LocalizationCatalogTests.swift to assert English baseline resolution.

Testing

• git diff --check was run and passed with no whitespace errors.
• pnpm check was attempted but failed due to an HTTP 403 when the environment tried to download tooling (SwiftFormat), so lint tooling did not complete.
• swiftformat Sources Tests and swiftlint --strict were not executed successfully in this environment because those binaries are not available.
• swift build and ./Scripts/compile_and_run.sh were attempted but failed because SwiftPM could not fetch external dependencies in this environment (HTTP 403), so an end-to-end build/test run could not be completed here.

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Codex Task