Fix charge items (Wizard Oil, Mana Oil, etc.) being treated as stacks

[daemonp]

Charge-based items (Wizard Oil, Mana Oil, Discombobulator Ray, Recombobulator, Deflector) were fundamentally broken in the Post tab. The addon treated charge counts as stack sizes, which meant:

• Stack Count was stuck at 1 — you couldn't batch-post multiple charge items
• Stack Size slider was shown but meaningless (you can't split/combine charges)
• Pricing, undercut, vendor price, and profit/loss calculations were all wrong
• The vendor-profit and bid-vendor-profit search filters overvalued charge items by a factor of their charge count