fix: Restore MSIX startup support, add Help menu and Windows startup settings button

[dixieflatline76]

Description

MSIX users lost the ability to start Spice with Windows after migrating away from the ISS installer. This PR restores startup support for MSIX via the desktop:StartupTask manifest extension, adds a "Manage in Windows Settings" button so all Windows users (MSIX and ISS) can manage startup behavior, adds a Help entry to the system tray menu pointing to the support page, and updates the support page with a user guide reference.

Changes

• MSIX Startup: Added desktop:StartupTask to msix/AppxManifest.xml (enabled by default, user-toggleable via Windows Settings)
• Settings Button: Added "Start with Windows" ButtonItem to General Settings (Windows-only) that opens ms-settings:startupapps
• Help Menu: Added "Help" tray menu item with help.png icon above "About Spice", linking to https://spicebox.dev/support.html
• Support Page: Added user guide reference paragraph before the "Open a Support Ticket" button
• Code Quality: Refactored config/const.go init() to use tagged switch on runtime.GOOS
• i18n: 4 new keys translated across all 10 supported languages

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

• Full make win-amd64 pipeline: gen-i18n ✓, linting ✓, all 44 test packages pass ✓
• Local MSIX sideload: verified Spice appears in Windows Settings → Startup Apps
• Verified "Manage in Windows Settings" button opens ms-settings:startupapps correctly
• Verified Help menu item opens support page in default browser
• Verified button is hidden on non-Windows platforms via runtime.GOOS guard
• make check-i18n passes with all 10 languages translated

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes