docs: add Ubuntu build and desktop integration guide

[katesclau]

Summary

• add a Ubuntu 26.04-tested build and installation guide
• document that the current release Makefile uses CMake and requires qtwebengine5-dev
• add a per-user desktop integration script for source builds and /opt installations
• link the Ubuntu guide from the README

Details

UBUNTU.md records the dependency and build findings from building the current master branch. It covers submodule initialization, the CMake-based release build, streaming-server setup performed by the Makefile, system-wide installation under /opt/stremio, and optional desktop registration.

The new desktop_integration.sh script creates or removes a user desktop entry below XDG_DATA_HOME/applications (or ~/.local/share/applications). It defaults to build/stremio and accepts STREMIO_EXECUTABLE=/opt/stremio/stremio for a system-wide installation.

Validation

• built successfully with make -f release.makefile on Ubuntu 26.04
• checked the generated executable shared-library resolution
• ran node --check on the downloaded server.js
• ran bash -n on desktop_integration.sh
• exercised desktop_integration.sh --install and --uninstall with an isolated XDG_DATA_HOME
• validated the generated entry with desktop-file-validate

Related issue

Relates to #500.