feat(plugins): add saved drafts support to plugins panel (#596)

[annukumar123]

Summary

Implements local draft input persistence in PluginsPanel for issue #596.

Key Changes

• Draft Persistence: Auto-saves text typed into the plugin input textarea to localStorage under localmind_plugin_draft_${sessionId}.
• Session Sync: Dynamically loads existing drafts when switching active session IDs.
• Execution Cleanup: Clears the saved draft from localStorage upon successful plugin execution.
• Test Coverage: Added unit test suite in PluginsPanel.test.jsx verifying draft restoration, real-time typing persistence, session switching, and post-execution cleanup.

Closes #596

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