fix validation lengths per schema

[dalers]

Attempted to edit imported product and the validation test for the description failed because it was (still) 50 characters.

Requested Copilot verify validation tests vs schema and implemented recommended changes.

Data validation conflicts in Stocks.php vs. stockmaster schema

Field	Code validation	Schema definition	Conflict
Description	Stocks.php:145: rejects if mb_strlen() > 50; HTML maxlength="50" (Stocks.php:1003)	varchar(255)	Overly restrictive — valid descriptions up to 255 chars (allowed by the schema) are rejected by a hard-coded 50-char limit.
BarCode	Stocks.php:176: rejects if mb_strlen() > 20, error message says "20 characters"; HTML maxlength="20" (Stocks.php:1305)	varchar(50)	Overly restrictive — legitimate barcodes of 21–50 chars (valid per schema, e.g. GS1-128/EAN longer formats) are rejected.
StockID	Stocks.php:157-168: only checks non-empty and ContainsIllegalCharacters(); no max-length check in PHP. HTML maxlength="20" (Stocks.php:918) is the only length limit, and it's client-side only	varchar(64)	Under-validated — server-side code never enforces the 64-char schema limit, and the only limit that exists (HTML maxlength=20) is far short of what the schema permits and easily bypassed (direct POST). A submitted StockID between 21–64 chars would be silently rejected by the UI, while one > 64 chars sent directly would hit the DB with no PHP-side check.

Other fields checked and found consistent (no conflict):

• Units — code limits to 20 chars, matches units varchar(20).
• DiscountCategory — HTML maxlength="2" matches char(2).
• CategoryID/TaxCat — populated from <select> lists sourced directly from stockcategory/taxcategories, so values are inherently constrained by the FK relationship.
• EOQ, Volume, GrossWeight, NetWeight, Pansize, ShrinkFactor — only numeric/positive checks are needed since the schema columns are double/decimal, and that's what's tested.

Recommended fix: relax the Description (50→255) and BarCode (20→50) length checks (and their HTML maxlength/error text) to match the schema, and add an explicit mb_strlen($StockID) > 64 check (with matching HTML maxlength="64") so the PHP validation actually enforces the schema's stockid varchar(64) limit.