cannot start duckdb UI

### What happens?

```sql
SELECT duckdb.install_extension('ui');
SELECT * FROM duckdb.raw_query($$ CALL start_ui(); $$);
```

fails with:

```
database-1 | 2025-08-31 10:02:57.227 UTC [191] ERROR: (PGDuckDB/CreatePlan) Invalid Input Error: Initialization function "ui_init" from file "/var/lib/postgresql/data/pg_duckdb/extensions/v1.3.2/linux_arm64/ui.duckdb_extension" threw an exception: "Failed to create directory "/var/lib/postgresql/.duckdb/extension_data": No such file or directory"
database-1 | 2025-08-31 10:02:57.227 UTC [191] STATEMENT: SELECT * FROM duckdb.raw_query($$ CALL start_ui(); $$)

```

### To Reproduce

see the code above

### OS:

MacOS

### pg_duckdb Version (if built from source use commit hash):

52ba7aa

### Postgres Version (if built from source use commit hash):

postgres:17.5-bookworm

### Hardware:

M2 macbook

### Full Name:

Georg Heiler

### Affiliation:

Complexity Science Hub

### What is the latest build you tested with? If possible, we recommend testing with the latest nightly build.

I have tested with a source build

### Did you include all relevant data sets for reproducing the issue?

Yes

### Did you include all code required to reproduce the issue?

- [x] Yes, I have

### Did you include all relevant configuration (e.g., CPU architecture, Linux distribution) to reproduce the issue?

- [x] Yes, I have

Start with the SQL reproduction using duckdb.install_extension('ui') and CALL start_ui(), then trace the ui_init failure and how the extension data directory is initialized. Done means the UI starts without the missing /var/lib/postgresql/.duckdb/extension_data error on the reported PostgreSQL, DuckDB, and ARM64 setup.

Written by the indexing model from the issue text.