feat(landing): scroll animation, battery sensors, chart spacing, footer polish (#57)

[gnacho]

Landing polish: scroll animation, battery sensors, chart spacing, footer rework

Five iterations, already deployed and visually validated by the user on
helios.cloudless.club (v8, md5 1dd0d7ca). Content identical to what is
live in production.

• Scroll animation for the "watch it breathe" section stats (theater stagger,
  respects prefers-reduced-motion).
• Centered battery widget + sensors (temperature, estimated autonomy).
• Chart spacing to use the available space.
• Landing copy fix: "Hecha para mirarla todos los días."
• Footer reduced; its starry-night effect moved to the "El sol no espera" CTA.
• Reordered end of page: screens -> CTA -> principles -> FAQ -> thanks
  (red heart) -> footer, with a section divider between FAQ and thanks.
• Dusk-to-night sky transition accelerated (non-linear phase curve).

Landing only; app/server untouched. Closes #57.