#146 Add per-tab remote indicator and Disconnect SSH tab context action

Adds the bonsplit half of cmux pane-scoped SSH connections (cmux ssh --pane):

• Tab/TabItem gain showsRemoteIndicator, rendered as a small network glyph after the tab title with accessibility text (localized en/ja).
• BonsplitController.updateTab/createTab/splitPane carry the new field.
• New TabContextAction.disconnectRemote context menu item ("Disconnect SSH", localized en/ja), shown only when the host's tabContextDisconnectRemoteAvailabilityProvider returns true.

Includes unit tests for the indicator field plumbing and context action availability.

Consumed by the cmux PR for pane-scoped SSH connections (link added there).

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Summary by cubic

Adds a per-tab remote connection indicator and a “Disconnect SSH” context action to support pane-scoped SSH sessions. This clarifies which tabs are remote and gives users a quick way to disconnect when the host allows it.

• New Features

  • New showsRemoteIndicator on tabs; renders a small network glyph after the title with accessible text (en/ja).
  • BonsplitController.createTab, updateTab, and splitPane accept and propagate showsRemoteIndicator.
  • New TabContextAction.disconnectRemote; visibility controlled by host via tabContextDisconnectRemoteAvailabilityProvider.
  • Unit tests for indicator plumbing and context menu availability.

• Migration

  • Pass showsRemoteIndicator when creating or updating tabs that are connected remotely.
  • Provide tabContextDisconnectRemoteAvailabilityProvider and handle the .disconnectRemote action to enable the menu item.

Summary by CodeRabbit

• New Features
  • Tabs can now display a remote SSH connection indicator icon when connected.
  • Tab context menu includes a new "Disconnect SSH" action when applicable.
  • Accessibility support added for remote connection status labels.
  • Japanese localization added for SSH-related menu items and labels.