fix(ios): adopt iOS 26 close, cancel, and confirm button roles in sheets

[zunda-pixel]

Closes #1524

iOS sheet dismiss and confirm buttons were custom flat buttons, inconsistent with the native iOS 26 controls used by system apps like Mail. iOS 26 adds semantic button roles that let the system render the Liquid Glass control (shape, material, spacing, typography) automatically.

Changes

• Add SemanticButtons.swift with CloseButton, CancelButton, and ConfirmButton. Each uses the iOS 26 role (.close / .cancel / .confirm) when available and falls back to a titled button on iOS 18.
• Adopt them across the sheets: connection form, insert row, filters, tag/group forms, tag/group management, FK preview, row detail, and the Settings sheet.
• Use CancelButton for the Cancel actions in alerts and confirmation dialogs, and for the inline Cancel on the connecting screen.

Notes

• Deployment target stays at iOS 18. The new roles are gated with if #available(iOS 26.0, *).
• On iOS 26 the role buttons use the system-provided presentation, so the explicit titles ("Done", "Cancel") only render on the iOS 18 fallback path.

Before	After

[github-actions]

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[datlechin]

LGTM, thank you.