refactor: replace sidebar, FAQ, and selection hardcoded colors with theme tokens

[aliworksx08]

This refactor removes remaining hardcoded colors from the sidebar and a few shared UI surfaces, replacing them with theme-backed tokens or shared CSS variables.

• Added sidebar.divider and sidebar.versionText to src/theme.ts
• Replaced hardcoded colors in src/components/layout/Sidebar.tsx
• Kept src/components/layout/GlobalSearchBar.tsx unchanged because it already uses theme tokens
• Replaced hardcoded FAQ border colors with theme border tokens in src/components/FAQ.tsx
• Replaced hardcoded global text selection colors with shared CSS variables in src/index.css

[anderdc]

• Drop the sidebar palette section. #3d3d3d is close enough to border.medium (rgba(255,255,255,0.2) ≈ #333), and #888888 is close enough to text.secondary (#7d7d7d). Two new palette entries plus TypeScript augmentations for one component's footer isn't worth it — use the existing tokens.
• Selection color changed. Original ::selection background was #031cdc, the PR changes it to #1d37fc. Different blue — keep the original or call out the visual change.
• Logo drop-shadow conversion is over-engineered. Converting <img> to <Box component="img"> with an sx theme callback just to resolve a static white glow (rgba(255,255,255,0.8)) isn't worth the added complexity. Leave as-is or use alpha('#ffffff', 0.8) without the component swap.

[anderdc]

Feedback addressed — sidebar palette dropped, selection color and logo changes removed. Clean swaps using existing tokens. Needs CI to pass before merge.