feat(ui): homepage restructure, left-align, theme toggle, EN/ES switch

[albertogg]

Summary

Refresh homepage UI/IA inspired by mariozechner.at while keeping current Hugo stack/i18n conventions.

Final decisions (single source of truth)

1) Homepage structure/order

1. Hero block
2. Nav bar
3. Who I am
4. What I offer
5. Notes (musings)
6. Get in touch

2) Alignment

• All homepage items/sections are left-aligned.

3) Icons

• Use Bootstrap Icons as the single icon source: https://icons.getbootstrap.com/
• Applies to social icons and UI icons (including dark/light toggle icon).

4) Theme toggle

• Add dark/light toggle icon button in nav.
• Place on the right side of nav.
• Persist user choice via localStorage.
• Fallback to prefers-color-scheme when no stored preference exists.

5) Language switch

• Add explicit language switch control in nav.
• Show target-language label only:
  • On English pages show ES
  • On Spanish pages show EN
• Use Hugo language-aware URLs and preserve page translation when available.

6) Content sources

• What I offer: i18n string content.
• Get in touch: social links rendered as Bootstrap Icons.

7) Typography

• Keep current serif font family for now.
• No font-family change in this issue.

8) Scope/intent

• Inspired by mariozechner.at structure/clarity, but implementation standardized for this repo.

Implementation plan (tiny slices)

• Slice 1: Add homepage section scaffolding in requested order.
• Slice 2: Apply left-alignment styles across homepage blocks.
• Slice 3: Add nav controls (theme toggle + language switch).
• Slice 4: Implement theme persistence and fallback behavior.
• Slice 5: Wire “What I offer” (i18n) and “Get in touch” icon links.
• Slice 6: QA + build validation.

Acceptance criteria

• Homepage order exactly matches requested sequence.
• All homepage content is left-aligned.
• Theme toggle works, is accessible, and persists.
• EN/ES switch is visible and behaves as target-language control.
• Build passes with hugo --gc --minify.
• No regressions in i18n/hreflang/RSS behavior.