Add user manual

[Copilot]

Creates a comprehensive user manual for the Premium Request Viewer app with step-by-step instructions and annotated screenshots for every feature.

New file: docs/user-manual.md

• Getting started — how to export the CSV from GitHub billing, upload via drag-and-drop or file picker
• Dashboard — KPI cards, filter bar (date range, username, model/org multi-select with chips), chart grid (daily trend, top users, model distribution, org breakdown)
• Quota exceedance — stacked bar chart and dedicated Exceedance Report page
• Data table — inline search, column sorting, pagination, CSV export
• User Detail page — per-user daily chart, model breakdown, SKU table, transaction list
• GitHub Profile Resolution — PAT token setup, rate limit display, localStorage storage note
• Theme toggle & new file workflow
• Data privacy — documents client-side-only processing, IndexedDB persistence, and the single external call to api.github.com

All 8 existing screenshots in docs/screenshots/ are referenced with correct relative paths.

Updated: README.md

Added a 📖 User Manual section pointing to docs/user-manual.md.

Original prompt

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This section details on the original issue you should resolve

<issue_title>User manual</issue_title>
<issue_description>Create a user manual for the app, include instructions and screenshots</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes User manual #4

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