feat: enable selecting flatpak packages on any distro when Flatpak is toggled on

[DsChauhan08]

Summary

Closes #46

When using a distro like Ubuntu, Fedora, or Arch, users can now toggle on Flatpak fallback to see and select apps that aren't available in the native repos but exist on Flathub. Selected flatpak fallback apps are installed alongside native packages — the generated command appends flatpak install after the distro's package manager, and the download script appends a dedicated flatpak section.

What changed

• New FlatpakToggle component — appears next to the distro selector for eligible distros (Ubuntu, Debian, Arch, Fedora, OpenSUSE)
• useLinuxInit hook — added isFlatpakEnabled state, toggleFlatpakEnabled, and isFlatpakFallback helper. isAppAvailable, toggleApp, selectAll, and availableCount now account for flatpak fallbacks when the toggle is on. State is persisted to localStorage.
• AppItem — shows a "flatpak" badge on fallback apps with Flatpak-blue checkbox color
• CategorySection — passes isFlatpakFallback through to AppItem
• CommandFooter — passes isFlatpakEnabled to script generation
• generateInstallScript — when isFlatpakEnabled is set, collects flatpak fallback packages and either appends them to the primary distro script or generates a standalone flatpak script
• flatpak.ts — new isAppended mode that generates a flatpak section meant to be appended to another distro's script (namespaced functions to avoid conflicts, increments TOTAL, reuses shared utils from the primary script)

How it works

1. User selects Ubuntu (or any eligible distro)
2. Toggles on "Flatpak fallback" in the header
3. Previously unavailable apps that have a Flatpak target now appear as selectable with a "flatpak" badge
4. The one-liner command gets && flatpak install flathub -y ... appended
5. The download script gets an appended flatpak section after the native installs