fix: remove list_concepts import that doesn't exist in history.py

The TUI app imported list_concepts from studyctl.history but the
function was never implemented. This caused an ImportError that was
masked by the textual try/except, showing "textual not installed"
even when it was.

Removed the import and stubbed the Concepts tab population.

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

Author: taylaand

packages/studyctl/src/studyctl/tui/app.py

@@ -23,7 +23,6 @@
 from studyctl.cli import TOPIC_KEYWORDS
 from studyctl.history import (
     get_study_session_stats,
-    list_concepts,
     spaced_repetition_due,
     struggle_topics,
 )
@@ -164,12 +163,8 @@ def _populate_concepts(self) -> None:
         table = self.query_one("#concepts-table", DataTable)
         table.add_columns("Name", "Domain", "Description")
 
-        for concept in list_concepts():
-            table.add_row(
-                concept["name"],
-                concept["domain"],
-                concept.get("description") or "",
-            )
+        # list_concepts not yet implemented — show empty table
+        pass
 
     def _populate_sessions(self) -> None:
         table = self.query_one("#sessions-table", DataTable)