Add Debian/Ubuntu preflight dependency check for build-shotcut.sh

[Proggerdogger]

Summary

scripts/build-shotcut.sh can fail late in a long build when a host tool is missing. This adds scripts/preflight-deps.sh for Debian/Ubuntu-family systems to report all missing packages in one sorted sudo apt install command, and runs it early from build-shotcut.sh by default.

• Exit 1 aborts the build with the install hint
• Exit 2 (unsupported OS) warns and continues
• Disable with ACTION_PREFLIGHT=0 in build-shotcut.conf
• Does not install packages or run apt

Test plan

• scripts/preflight-deps.sh exits 0 on a stock Debian/Ubuntu build host with deps present
• With tools removed from PATH, exits 1 and prints one sorted sudo apt install -y … line
• ACTION_PREFLIGHT=0 skips the check
• Unsupported OS path exits 2 and build continues with a warning

[ddennedy]

This is not a change I want to include because build-shotcut.sh is primarily used by our CI and maintained by the main developers who maintain and support the builds generated by CI .

[ddennedy]

I can accept the new script and mention it in the readme.

[Proggerdogger]

Sounds good. I can understand you not wanting to maintain a new script when it's not generally useful. Putting it in the readme means someone can make the install easier if they want to. It'll be interesting to see if anyone uses it.