docs(readme): fix Linux install instructions, package filenames, and repo.querya.app status #1500

Problem

The current README.md has several critical discrepancies and broken commands in the Installation section:

1. Unreachable APT/DNF repository: Lines 125-139 instruct users to install via https://repo.querya.app/ (curl -fsSL https://repo.querya.app/gpg.key ...). The domain repo.querya.app does not exist yet (curl: (6) Could not resolve host: repo.querya.app), because repository deployment (feat(packaging): setup automated APT and DNF repository for Debian/Ubuntu and Fedora #1059) is still pending. Users attempting to install following README get an immediate failure.
2. Incorrect .deb package filename & suboptimal command:
   • Line 144 mentions querya-desktop-linux-amd64.deb. The actual release asset generated by CI is Querya-Desktop-${VERSION}-linux.deb (e.g. Querya-Desktop-0.4.18-linux.deb).
   • Line 144 uses sudo dpkg -i ... && sudo apt-get install -f. Recommending sudo apt install ./Querya-Desktop-*-linux.deb is standard and resolves dependencies automatically.
3. Missing libfuse2 requirement for AppImage:
   • Modern Ubuntu (22.04 / 24.04+) does not ship libfuse2 by default; AppImages fail to launch without libfuse2 / libfuse2t64.
4. Missing Arch Linux (AUR) and Flatpak documentation:
   • Arch Linux package querya-desktop (yay -S querya-desktop) and local Flatpak bundle install (flatpak install --user ./Querya-Desktop-*-linux.flatpak) are supported in CI but omitted from README.
5. macOS Gatekeeper tip:
   • Mention xattr -cr /Applications/Querya.app for unsigned community builds.

Scope

• In README.md:
  • Demote/mark repo.querya.app as "Coming Soon" (reference feat(packaging): setup automated APT and DNF repository for Debian/Ubuntu and Fedora #1059) and promote GitHub Release assets and AUR as the primary installation methods.
  • Fix .deb filename pattern and use sudo apt install ./Querya-Desktop-*-linux.deb.
  • Add AppImage prerequisite for Ubuntu 22.04+ (libfuse2 / libfuse2t64).
  • Add Arch Linux (AUR: yay -S querya-desktop) to README.
  • Add local Flatpak instructions (flatpak install --user ./Querya-Desktop-*-linux.flatpak).
  • Add macOS unsigned build bypass note.

Acceptance

• Commands in README for Debian/Ubuntu, Fedora, Arch, AppImage, and macOS work without errors when tested against release assets.
• No references to unresolved domains as active installation methods.