English default locale, tr.lproj bundle, macOS 15/16 compat, misc fixes

[hesit8]

Summary

• macOS compatibility — platform target changed from .v26 (macOS 16 beta SDK) to .v15 minimum, supporting macOS 15 and 16
• English as default language — app was defaulting to Turkish on non-Turkish systems; English is now the default with Turkish available as a proper locale selection
• Standard Apple localisation — Turkish strings moved from hardcoded L("tr", "en") pairs into tr.lproj/Localizable.strings; defaultLocalization = "en" added to Package.swift
• Date formatter locale — hardcoded tr_TR locale in RateLimitFetcher replaced with en_US_POSIX so month/time labels display correctly for all users
• Close button fix — the Close button on the account-added success screen inside the menu popover was calling closeAddAccountWindow() which is a no-op in that context; it now resets addingStep to idle correctly
• Fetch failure backoff bug — consecutiveFetchFailures backoff would permanently stop rate limit polling after 3 consecutive failures (counter never reset); now backs off for 3 cycles then retries
• Hardcoded Turkish strings — fallback strings in ProfileManager and AppStore replaced with English
• Sparkle auto-updates disabled — SUEnableAutomaticChecks set to false; users who build from source can review changes before manually updating
• Build script — Sparkle.framework now bundled into app and rpath fixed so the app launches correctly

Test plan

• Build with swift build -c release on macOS 15
• Run bash build.sh and open CodexSwitcher.app — should launch and appear in menu bar
• Add an account — Close button on success screen should dismiss back to normal menu
• Verify all UI strings appear in English by default
• Set appLanguage to tr in UserDefaults and verify Turkish strings load from bundle

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