Updated permissions model and non-root support

[alexhb1]

• Adds a non-root startup path at user 1000:1000 - skips privilege escalation and ownership checks. Works e.g. for kubernetes deployments (user 1000:1000 and runAsNonRoot enabled).
• Remove startup check/chown commands for user-owned folders. Checks can be done with a "Test destination" button in settings which performs a test write. Users are responsible for fixing their own permissions.
• Update docs