feat: add fluency score news banner and update stats handling in webview

[rajbos]

This pull request introduces a new "Fluency Score" news banner to inform users about the Copilot Fluency Score dashboard, improves the update mechanism for the usage analysis panel to preserve repository hygiene results, and enhances the user experience by retaining webview context when switching tabs. The most important changes are grouped below:

Fluency Score News Banner and User Tracking:

• Added a new method showFluencyScoreNewsBanner to display a one-time banner introducing the Copilot Fluency Score dashboard, shown after the extension has been opened five times unless the user has already visited the dashboard or dismissed the banner. The banner offers options to open the dashboard or dismiss the message. (src/extension.ts) [1] [2] [3]
• Updated the showMaturity method to record when a user opens the Fluency Score dashboard, preventing further display of the banner. (src/extension.ts)

Usage Analysis Panel Improvements:

• Changed the update logic for the analysis panel to use postMessage instead of directly setting HTML, allowing for partial updates and preserving repository hygiene results. The webview now receives a structured message with updated stats and locale information. (src/extension.ts, src/webview/usage/main.ts) [1] [2]

Webview Experience Enhancements:

• Set retainContextWhenHidden: true for the analysis panel webview, ensuring that analysis results persist when switching tabs and improving performance and user experience. (src/extension.ts)