chore(ci): Add flatter to host Flatpak repo on GitHub Pages (#76)

* chore(ci): Add flatter to host Flatpak repo on GitHub Pages

- Update release.yml to not upload .flatpak file to releases
- Create flatter.yml to automate building and hosting of Flatpak via GitHub pages using andyholmes/flatter
- Add nightly branch for main pushes and stable branch for releases
- Update README.md with the new Flatpak installation instructions

Co-authored-by: arabianq <55220741+arabianq@users.noreply.github.com>

* Potential fix for pull request finding 'CodeQL / Workflow does not contain permissions'

Co-authored-by: Copilot Autofix powered by AI <62310815+github-advanced-security[bot]@users.noreply.github.com>

---------

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>
Co-authored-by: Copilot Autofix powered by AI <62310815+github-advanced-security[bot]@users.noreply.github.com>

README.md

@@ -52,6 +52,28 @@ three main components:
 You can download pre-built binaries and .deb packages from
 the [releases page](https://github.com/arabianq/pipewire-soundpad/releases).
 
+## **Flatpak**
+
+You can install PWSP via Flatpak from our custom repository hosted on GitHub Pages.
+
+Add the repository:
+
+```bash
+flatpak remote-add --user --if-not-exists arabianq-repo https://arabianq.github.io/pipewire-soundpad/index.flatpakrepo
+```
+
+Install the stable version:
+
+```bash
+flatpak install --user arabianq-repo ru.arabianq.pwsp//stable
+```
+
+Or install the nightly version (latest commit to `main`):
+
+```bash
+flatpak install --user arabianq-repo ru.arabianq.pwsp//nightly
+```
+
 ## **Fedora Linux (and derivatives)**
 
 If you're using Fedora, you can install PWSP from a dedicated repository using DNF.