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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