chore: establish safe preview-to-main delivery flow

[ZoltanWolf]

What

• add documented Git preview/release flow (docs/GIT_PREVIEW_RELEASE_FLOW.md)
• add script to apply branch protection on main (scripts/setup_github_branch_protection.sh)

Why

• keep main stable while Register-First is developed and previewed
• enforce PR-first delivery instead of direct pushes

Notes

• this PR intentionally does not touch product behavior
• existing local uncommitted workspace changes are not included in this commit

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[ZoltanWolf]

Follow-up needed by a repo admin:

Branch protection for main cannot be set from this token (permission is write, not admin).

Run after checkout:

cd /Users/zoltangal/Desktop/METAProjekt/Programmieren/studio
./scripts/setup_github_branch_protection.sh Egonso/studio main

This enforces PR-first flow on main.

[ZoltanWolf]

Update: the branch-protection script now supports both modes:

1. PR protection only (no required checks)

cd /Users/zoltangal/Desktop/METAProjekt/Programmieren/studio
./scripts/setup_github_branch_protection.sh Egonso/studio main

2. PR protection + required checks

cd /Users/zoltangal/Desktop/METAProjekt/Programmieren/studio
./scripts/setup_github_branch_protection.sh Egonso/studio main "lint" "typecheck"

Note: this still must be executed by a repo admin account.