feat: upgrade Flatpak runtime to 25.08

[zakweb3]

Description

• Bumps the Flatpak runtime (org.freedesktop.Platform) from the deprecated 23.08 to 25.08.
• Updated flatpak.baseVersion and runtimeVersion in electron-builder.yml from '23.08' to '25.08'
• Adds dedicated build:flatpak npm script for building .flatpak bundles
• Generates bundle at dist/open-webui.flatpak (~92 MB)
• I installed/tested the generated the flatpak bundle on my laptop and it is working properly.
• The next step would be testing this change on the CI/CD Github pipeline.

Install required runtime and SDK

sudo apt-get update && sudo apt-get install -y flatpak flatpak-builder 
flatpak --user remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
flatpak --user install -y flathub org.freedesktop.Platform//25.08 org.freedesktop.Sdk//25.08 org.electronjs.Electron2.BaseApp//25.08 

Build Process

 npm install
 npm run build:flatpak

Related Issues

• The main issue this PR resolves for me was the dependency on the deprecated 23.08 runtime, I didn't wanted to install that dependency.
• The size of the Flathub version of this app for some reason is around 800 MB, I didn't wanted to download that.

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[NN708]

FYI, we already have a manifest on Flathub that uses the Freedesktop SDK 25.08: https://github.com/flathub/com.openwebui.open-webui/blob/master/com.openwebui.open-webui.yml

[zakweb3]

FYI, we already have a manifest on Flathub that uses the Freedesktop SDK 25.08: https://github.com/flathub/com.openwebui.open-webui/blob/master/com.openwebui.open-webui.yml

Do you know why the flathub bundle is 781 MiB? Also, on flathub it shows the version is 0.0.8!
I am not an expert on this, but the npm flatpak build process seems simple and lean (~92 MB).
I am looking forward to improve the pipline from the npm flatpak build release to the flathub build release workflow.
Flathub is a major Linux Desktop app discovery and distribution, so it is important to get the best release workflow on that platform.

[NN708]

According to Flathub's requirements, apps must be built from source. If you have ideas for optimizing the size, feel free to open a pull request in the flathub/com.openwebui.open-webui repository.

Regarding the version: the actual package is up to date, but the version displayed on the Flathub website is pulled from com.openwebui.open-webui.metainfo.xml. Updating that file should resolve the mismatch.