fix: learn/check-update/update commands no longer require TuiService (#542)

Move stateless commands (Learn, CheckUpdate, Update) before TuiService::new()
in run_offline_command(), matching the existing Guard early-return pattern.
Extract learn command handler into standalone run_learn_command() function,
eliminating code duplication across offline and server command paths.

Change embedded config KG path from relative "docs/src/kg" to absolute
self.get_default_data_path().join("kg"), matching build_default_desktop().

Closes #542

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: AlexMikhalev