Introduce UNICEF blue as primary brand color

[amoghbanta]

Summary

• Replace achromatic --primary CSS variable with UNICEF blue (#1CABE2 / oklch(0.62 0.14 230)) in both light and dark themes, cascading the brand color to all buttons, badges, focus rings, and active states app-wide
• Replace hard-coded blue-* Tailwind classes with semantic primary tokens in StepIndicator, storyboard rebuild banner, page edit hint card, and LLM logs step tracker
• Keep green for completion states: "Rendered" and "Done" badges now use bg-green-100 text-green-700 instead of the (now-blue) default badge variant
• Add micro-UX polish: solid blue "Get Started" CTA, blue app title, book card hover lift animation, stronger section header backgrounds (bg-muted/50), primary-tinted empty state hover

Test plan

• pnpm typecheck — clean
• pnpm vitest run — all 289 tests pass
• Visual check: blue buttons, stepper, sidebar selection, rebuild banner, hint card
• White text on blue buttons is clearly readable (AA contrast)
• Green badges still show for Rendered/Done states
• Dark mode: lighter blue variant renders correctly