feat: add light mode

[pmusaraj]

Add a light mode theme that users can toggle from the profile dropdown menu (avatar click). Preference persists via localStorage.

Changes:

• New ThemeContext manages dark/light state and toggles .dark/.light class on the document root
• Light mode CSS variables for all theme colors (backgrounds, text, borders, scrollbars, diff highlights, markdown syntax highlighting)
• Replaced hardcoded dark hex colors in diff views with CSS variables so insert/delete/comment-range backgrounds adapt to theme
• Replaced hardcoded bg-[#1a1a1a] and bg-[#0d1117] in app-shell and pr-overview with semantic classes (bg-muted, bg-card)
• Sun/Moon toggle added to the user menu

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