chore: remove xlsx export pro gate

[datlechin]

Summary

• XLSX export is now free for all users.
• Removed the Pro gate from the Export dialog ((Pro) suffix in the format Picker, the orange "XLSX export requires a Pro license." caption, the Activate License... link, and the isExportDisabled short-circuit on unlicensed Macs).
• Removed the supporting machinery: proFormatIds set, isProGatedFormat(_:) predicate, the now-orphaned showActivationSheet @State + .sheet modifier, the xlsxExport case in ProFeature (plus its 3 switch arms), the unused localized strings ("XLSX Export", "XLSX export requires a Pro license."), and the docs Note in import-export.mdx.
• License-state changes no longer affect Excel export at all.

Test plan

• Build TablePro and open Export dialog without an active Pro license.
• XLSX appears in the format Picker without a (Pro) suffix.
• Selecting XLSX shows row/table count, not the orange Pro warning.
• Export button is enabled for XLSX (assuming valid filename and rows to export).
• Activate a Pro license and re-check: same behavior, no regression for the still-gated features (iCloud Sync, Encrypted Export, Env Var References, Linked Folders).
• Verify swiftlint lint --strict passes.

[chatgpt-codex-connector]

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