Desktop app missing /unshare command #8602

Description

Issue Description

The desktop version of OpenCode is missing the /unshare command that exists in the TUI version.

Current State

TUI (working):

• Has /unshare command that removes share links and makes sessions private
• Command is documented in tui.mdx and functional

Desktop/Web App (missing):

• Has share functionality (can share sessions via session header UI)
• NO /unshare slash command
• NO unshare button in the UI
• Users can share sessions but cannot unshare them through the interface

Expected Behavior

Desktop app should have the same unshare functionality as the TUI version:

1. /unshare slash command should be available in the command palette
2. Optionally, an unshare button in the session header UI

Impact

Users who share sessions from the desktop app cannot revoke access through the UI, which is a privacy/usability concern.

Steps to Reproduce

1. Use the desktop app
2. Try to run /unshare command
3. Command is not available in the slash commands list (only /share is available)

Additional Context

This creates a feature discrepancy between the TUI and desktop interfaces. The underlying API endpoint DELETE /session/:id/share exists and is used by the TUI version (see packages/opencode/src/cli/cmd/tui/routes/session/index.tsx:396-407).

The desktop app already has share functionality implemented in packages/app/src/components/session/session-header.tsx but lacks the corresponding unshare capability.

Plugins

None

OpenCode version

1.1.20