fix: Homebrew 6.0+ compatibility, startup Keychain prompts, and WebView timezone offset parsing - #152

This pull request resolves three distinct classes of bugs affecting both the Svelte/Tauri desktop app and the Swift native SwiftUI wrapper:

1. Homebrew 6.0+ Built-in brew vulns Compatibility

• Issue: The original codebase assumed brew-vulns was always installed as a standalone formula (homebrew/brew-vulns/brew-vulns). In Homebrew 6.0+, the vulns subcommand has been integrated natively into core Homebrew, and the standalone tap/formula has been deprecated and deleted. As a result, detection probes (brew --prefix brew-vulns) failed, and clicking "Install" attempted to fetch a deleted formula, failing indefinitely.
• Fix:
  • Updated detection logic on both the Rust (check_brew_vulns_installed) and Swift (isBrewVulnsInstalled) backends to check brew help vulns as a fallback when the legacy prefix check fails.
  • Changed the installation command/remediation to brew update instead of brew install homebrew/brew-vulns/brew-vulns since the subcommand is natively bundled with current Homebrew.
  • Updated validate_formula_name in both Rust and Swift to accept 2-segment tap-qualified formula names (e.g., user/formula like anomalyco/opencode), which are fully valid.

2. Intrusive Startup macOS Keychain Prompts

• Issue: On app launch, github.loadStatus() was called eagerly. If a user had not configured credentials or was not authenticated, macOS would trigger an intrusive Keychain access prompt on launch.
• Fix: Added a persistent brew-browser:github:signed-in flag in localStorage when a user successfully authenticates. Eager startup check calls to github.loadStatus() are now gated on this flag, preventing Keychain checks on startup for unauthenticated users.

3. Last Scan "59 Minutes / 1 Hour Ago" WebView Parsing Bug

• Issue: Under WebKit (macOS Tauri webview), parsing ISO-8601 date strings with nanosecond precision (e.g., "2026-07-16T16:50:00.123456789Z" returned by the Rust backend) has a timezone parsing bug where it fails or ignores the Z suffix and parses the time in the host system's local timezone. This resulted in the relative scan time showing as exactly "59 minutes ago" or "1 hour ago" immediately after a successful scan.
• Fix: Added a parseIsoDate utility helper in Svelte (vulnerabilities.svelte.ts) to truncate timestamps to standard millisecond-precision before parsing them into JavaScript Date objects.

4. Process Environment Fix for SwiftUI Subprocesses

• Issue: In the Swift native app wrapper, VulnsService spawned its subprocesses (like check and scan commands) without inheriting the correct Homebrew environment (specifically missing $HOME), causing brew to fail to execute correctly under launchd GUI app bundle contexts.
• Fix: Set process.environment = BrewService.brewEnvironment() in VulnsService.run to align environment variables consistently with the rest of the native app.