Add --audience flag to share-link create and update

[AndreyVMarkelov]

Summary

• Add --audience flag to share-link create and share-link update
• Supported values: public, team, members, no-one
• On create: applies audience to new links and modifies existing links via ModifySharedLinkSettings
• On update: includes audience in the "at least one flag required" validation
• Reuses shared shareLinkAudienceFlag parser across both commands

Depends on #256.

Test plan

• share-link create --audience team sets audience on new link
• share-link create --audience members modifies existing link
• Invalid --audience value returns clear error before API call
• share-link update --audience no-one sets audience via modify
• Invalid audience on update returns clear error
• Flags are registered on both commands
• golangci-lint clean, all tests pass