buckaroo.server hard-imports polars, forcing polars install for pandas/xorq-only tauri users

[paddymul]

Problem

buckaroo/server/data_loading.py:5 does import polars as pl at module top:

```python
import pandas as pd
import polars as pl # ← unconditional
from buckaroo.serialization_utils import to_parquet, pd_to_obj, check_and_fix_df
```

This propagates upward — buckaroo.server.app, buckaroo.server.handlers, and ultimately python -m buckaroo.server all fail to import without polars installed:

```
ModuleNotFoundError: No module named 'polars'
```

For the buckaroo-tauri integration, this means users who only want to view pandas (or xorq) data are forced to install polars. That's wrong: pandas is structurally required (the widget materializes pandas frames internally), but polars should be optional, just like xorq is.

Surfaced while building xorq-desktop's integration. The Tauri sidecar should start on a pandas-only install; today it can't.

Polars usage in data_loading.py is already isolated to the lazy codepath

pl.* is referenced only at lines 113-189 — load_file_lazy, get_metadata_lazy, get_display_state_lazy, handle_infinite_request_lazy. Everything above (the pandas codepath) doesn't touch polars. So the import is structurally optional with zero refactor needed:

```
$ grep -nE "\bpl.|polars" buckaroo/server/data_loading.py
5:import polars as pl
113:def load_file_lazy(path: str) -> pl.LazyFrame:
117: return pl.scan_parquet(path)
…
185: select_exprs = [pl.col("index")]
188: select_exprs.append(pl.col(orig).alias(rw))
```

Precedent: buckaroo already has the pattern

Three existing modules use try: import polars for the same reason:

• buckaroo/__init__.py:115-116
• buckaroo/marimo_utils.py:38-39, 50-51
• buckaroo/artifact.py:100-101, 153-154

data_loading.py should follow the same idiom.

Suggested fix

Option A (minimal — matches the marimo_utils pattern): move import polars as pl to a function-local import inside each *_lazy function. The non-lazy codepath continues to import the module successfully without polars.

Option B (slightly cleaner — matches the __init__.py pattern): module-level guarded import + a sentinel:

```python
try:
import polars as pl
HAS_POLARS = True
except ImportError:
pl = None
HAS_POLARS = False
```

…then raise a clear ImportError("polars required for *_lazy paths; pip install polars") at the top of each *_lazy function. Better user-facing message than the deep stack on first call.

Either way the fix is ~10 lines.

Follow-up: [tauri-essentials] extra (#753)

This bug is the reason #753's [tauri-essentials] extra currently pulls polars. Once data_loading.py is fixed, that extra should drop polars>=1.24.0 from its required list — polars-using buckaroo-tauri consumers would add buckaroo[tauri-essentials,polars] themselves.

I'll hold off rebasing #753 until the direction here is settled. If you'd prefer #753 land first and then this fix follow, that works too — they're independently mergeable.