parts table: supplier and supplier part number columns

[samyk]

The parts table could show the stock at a supplier, but not which suppliers a part has or their order numbers, and it could not be sorted by either.

This adds two columns, both hidden by default so the stock table is unchanged:

• Supplier: the distinct supplier names from the part's order details, comma-separated, each linked to that supplier's parts list (like the category and manufacturer columns).
• Supplier part number: the non-empty supplier part numbers, comma-separated, with the supplier name as tooltip. A number links to the supplier product URL (new tab) when that URL is http(s).

Both are available in the column chooser and in the default-columns setting (supplier, supplier_part_number in TABLE_PARTS_DEFAULT_COLUMNS; documented in docs/configuration.md).

Both sort server-side in the filter query, by NATSORT(MIN(_suppliers.name)) and NATSORT(MIN(NULLIF(_orderdetails.supplierpartnr, ''))), using the existing _orderdetails/_suppliers join and group-by-part, as the storage location column does. Parts without a supplier (or with only empty numbers) sort last in both directions. The detail query fetch-joins orderdetails.supplier, so rendering does not lazy-load suppliers.

Tested on sqlite with ~18k parts and ~18k order details: sorting by either column takes about the same time as sorting by name. MySQL and PostgreSQL run in CI but the new sort was not exercised there by hand.

Notes for review:

• NULLs-last is done with a two-expression orderField (a CASE WHEN MIN(...) IS NULL followed by the NATSORT one), relying on the adapter appending the direction to the last expression only. Happy to move that into an explicit order-by if you prefer.
• When a list is already restricted on the order details join (e.g. a supplier's own parts list), the sort uses the minimum over the matching order details only.
• The BOM table was left alone.

[codecov]

Codecov Report

❌ Patch coverage is 85.36585% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.47%. Comparing base (a7f2d49) to head (2e7e85f).

Files with missing lines	Patch %	Lines
src/DataTables/Helpers/PartDataTableHelper.php	78.57%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #1574   +/-   ##
=========================================
  Coverage     64.46%   64.47%           
- Complexity    10210    10217    +7     
=========================================
  Files           762      762           
  Lines         32711    32752   +41     
=========================================
+ Hits          21087    21116   +29     
- Misses        11624    11636   +12     

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