fix: add macOS helper extension support

[GrantWasil]

Summary

• Detect LuaJIT's OSX platform explicitly and use Steam's macOS browser-profile path.
• Sign Chromium tracked preferences on macOS using the local IOPlatformUUID and Steam CEF's empty HMAC seed.
• Treat Secure Preferences as authoritative on macOS, validate its extension/developer-mode/super MACs, and safely re-sign cleanup changes.
• Replace the Linux shutdown path with a detached macOS restart targeting only Steam Helper and steam_osx.
• Add macOS paths and relaunch guidance to the README.
• Keep the contributor-facing installer in sync and add canonical HMAC golden tests.

Root cause

The installer treated every non-Windows platform as Linux. On macOS this selected ~/.steam/steam/config/htmlcache/Default, invoked Linux-only process/UI commands, and skipped Chromium preference signing.

There was also an existing tracked-preference bug: the developer-mode MAC was written to protection.ui.developer_mode. Chromium stores path hashes beneath protection.macs, so the correct location is protection.macs.extensions.ui.developer_mode.

User impact

With a macOS-capable Millennium build, Extendium can now locate the real Steam CEF profile and install its helper extension with valid tracked-preference signatures. Failed or stale helper entries no longer produce a false installed state from Preferences alone.

The persistent machine identifier is read locally only for Chromium's HMAC calculation and is never logged or transmitted.

Validation

• bun test — 3 passed, including developer-mode and super-MAC golden vectors plus canonical serialization.
• Lua syntax parsing passed for all modified backend modules.
• Focused TypeScript checking passed for the installer and its tests.
• The implemented HMAC functions reproduced a valid extension MAC and the super MAC from this Mac's existing Steam Secure Preferences in a read-only check.
• An isolated fixture exercised install, status validation, Preferences-only rejection, invalid-path cleanup, unrelated-MAC preservation, re-signing, failure-before-write, and macOS restart command construction.
• millennium-ttc reported a completed production bundle with the repository's existing circular-dependency warning; its process retained an open event loop and was stopped after completion.
• bun run lint still reports the pre-existing eslint.config.mjs:19 unsafe-assignment error. That file is unchanged by this PR.

Dependency

This fixes Extendium's side of macOS support. Testing inside Steam still requires a macOS-capable Millennium build; upstream restoration is currently tracked in SteamClientHomebrew/Millennium#841.

Closes #113