[Task](ui): rename TabBar appearance="content" to "tabs" #2077

Task Description

As specified in #2028, the current appearance="content" on TabBar should be renamed to appearance="tabs", which becomes the new default value. "content" should remain functional but be marked as deprecated in favour of "tabs".

Scope

• Add "tabs" to TabBarAppearance type
• Change default from "main" to "tabs" in TabBar
• Normalize appearance="content" → "tabs" at runtime for backward compatibility
• Mark "content" as deprecated in JSDoc
• Update isContent check in TabBarItem to use normalized value
• Update stories and tests

Related

Part of #2028. Companion to #2020 (which adds appearance="buttons").