fix: prevent max_file_size from being set to 0

[echobt]

Description

This PR fixes a bug where setting max_file_size to 0 in the configuration was silently accepted, causing all files to be skipped during indexing.

Changes

• Added validation in commands.rs to reject 0 as a value for max-file-size.
• If a user attempts to set it to 0, vgrep now returns an error: Max file size must be greater than 0.

Verification

Reproduced the issue by running:

vgrep config set max-file-size 0

Before the fix, this succeeded. After the fix, it returns an error and prevents the invalid configuration.

Related to bounty issue #204.