Add per-item notes and custom columns

[H1D]

Summary

• Per-item notes: toggle-controlled notes field on each line item, rendered as italic sub-rows in the PDF table
• Custom columns: dynamically add/remove columns (e.g. HS codes for export) via "+ Add Column" button, rendered in the PDF between GTU and Amount
• Both features off by default — existing invoices from localStorage and shared URLs load unchanged
• 6 new URL compression keys (3–8) for lossless round-trip via share links

Test plan

• Open app, verify existing invoices load unchanged from localStorage
• Toggle "Notes" on, add per-item notes, verify italic sub-rows in PDF
• Click "+ Add Column", enter "HS Code", fill values, verify column in PDF table
• Toggle custom column off, verify it hides from form and PDF
• Remove a custom column via x button, verify cleanup
• Share URL, open in new tab, verify notes + custom columns restore
• Reset an item (single item), verify notes + custom fields clear

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Deploy Preview: https://699883eff9a0879dd955b81c--easypdf-lite.netlify.app

[DyArt]

Thanks, works perfectly.