NSIS: support admin vs per-user installs

[michelu89]

Update installer.nsi to avoid unnecessary UAC prompts and correctly handle admin vs standard-user installs. RequestExecutionLevel is set to "highest" and install/uninstall logic now detects account type to choose SetShellVarContext and to write Add/Remove Programs entries to HKLM (admin) or HKCU (per-user). Removed unnecessary icacls calls and ensure the uninstall registry key includes DisplayIcon/InstallLocation and NoModify/NoRepair values. Uninstaller now removes the entry from the same hive it was written to. Added robust previous-install detection across HKLM native, HKLM WOW6432Node and HKCU, and fixed file-size calculation by replacing FileSeek with a System/GetFileAttributesEx-based FileSizeNew to handle files >2GB. Also corrected the disk-space section to use the SEC_SPACE constant.

Description

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Fixes #(number of issue in GitHub)

Type of change

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works

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