fix: 9 security hardening fixes

[khalidelmerrah]

Summary

Security review of windows_fixer.py found 9 vulnerabilities ranging from critical (command injection) to low (thread safety). All fixes are in a single file with no new dependencies, no UI changes, and no breaking changes.

Fixes

#	Severity	Fix
1	Critical	Command injection in CHKDSK fix mode - removed cmd /c string interpolation, use list form + drive letter regex validation
2	Critical	Argument injection in admin relaunch - replaced manual quoting with subprocess.list2cmdline
3	High	Version tag from GitHub API sanitized to only allow v0-9. characters
4	Medium	Settings file validated after loading - type enforcement + language whitelist
5	Medium	resource_path() rejects .. and absolute path prefixes (path traversal)
6	Low	shutil.rmtree uses onerror handler instead of ignore_errors=True - logs failed deletions
7	Low	Critical functions use specific exception types instead of bare except: pass
8	Low	CommandRunner cancel/skip flags use threading.Event instead of bare booleans
9	Low	BUILD_DATE reads from env var with fallback (accurate in CI builds)

Details

Fix 1 (Command Injection): step_chkdsk() previously used ["cmd", "/c", f"chkdsk {drive} /f"] which allows shell injection if the drive string is manipulated. Now uses ["chkdsk", drive, "/f"] with regex validation [A-Z]:.

Fix 2 (Argument Injection): relaunch_as_admin() wrapped args in f'"{a}"' without escaping embedded quotes. Now uses subprocess.list2cmdline() which handles all edge cases.

No new dependencies. No UI changes. No breaking changes.

Test plan

• Verify app launches normally
• Test CHKDSK in both scan and fix mode
• Test "Run as Admin" button
• Test "Check for Updates"
• Verify settings save/load works
• Test cancel and skip during operations