Side panel scaffold

[Omoshirokunai]

The side panel is an optional panel that opens alongside Google Docs.
It is the interface for Ollama and Gemini API interactions.

Acceptance criteria

• Side panel opens from a button in the popup
• Panel persists while the Google Docs tab is open
• Panel has placeholder layout: prompt field, model selector,
  response area, Accept/Modify/Reject buttons (non-functional)
• Panel state survives popup open/close cycles

Notes

test sing chrome.sidePanel API (MV3).
The panel does not need to make any API calls in this issue.