Use AOI bounds for FCF clip_box (fix WindowError on some non-CONUS AOIs)

[ZachHoppinen]

Summary

• Some non-CONUS AOIs (e.g. white_mtns 2024_2025) fail at `generate_forest_fraction_dataarray` with `rasterio.errors.WindowError: Bounds and transform are inconsistent` when `fcf.rio.clip_box(...)` is called with bounds derived from `ref.rio.bounds()`.
• Investigation: `generate_reference_grid` already reprojects `ref` to EPSG:4326 (line 130) and clips it to `aoi.bounds` (line 137). So `ref.rio.bounds()` ≈ `aoi.bounds` — the "tighter crop" benefit from Fix OOM when downloading Proba-V forest cover for non-CONUS AOIs #80 was illusory.
• Pixel-edge snapping in `pad_box` and projection round-tripping in `reproject` can produce subtly degenerate bounds that break `rasterio.windows.from_bounds`. Bypass the issue by using `aoi.bounds` directly.
• Also removes the now-unused `transform_bounds` import.

Test plan

• `pytest tests/` — 73/73 pass.
• Re-run white_mtns 2024_2025 and confirm FCF clip succeeds.
• Confirm CONUS AOIs still work (NLCD path is untouched).

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