feat: Build Philosophy, Architecture Preview, and Footer sections

[100gle]

Parent Issue

Part of #2 — Phase 2: Hero Landing Page

Objective

Build the remaining landing page sections: Philosophy storytelling, Architecture preview, and site Footer.

Context — Philosophy

Bub was born in group chats, not as a demo. Key messages:

• "Born in Group Chats, Not Demos"
• Most agent systems work for single-user demos but fail in shared team environments
• Bub is designed for operator equivalence — humans and agents share the same model
• Link to blog post: "Socialized Evaluation and Agent Partnership"

Context — Architecture

The turn lifecycle pipeline:

resolve_session → load_state → build_prompt → run_model
                                                    ↓
              dispatch_outbound ← render_outbound ← save_state

• Each stage is a pluggy hook
• ~200 lines of core code
• Builtins are ordinary plugins — override any stage
• Link to deep-dive: /en/concepts/architecture/

Tasks

• Create src/components/Philosophy.astro
  • Section title: "Born in Group Chats, Not Demos"
  • Two-column layout: text left, visual/illustration right (placeholder div is fine)
  • Key points as brief paragraphs (not bullet list)
  • CTA link: "Read about Socialized Evaluation →"
• Create src/components/ArchitecturePreview.astro
  • SVG or styled-div pipeline diagram showing the hook stages
  • Brief text explanation (2-3 sentences)
  • CTA link: "Deep Dive into Architecture →"
  • Diagram should be responsive (scale down on mobile)
• Create src/components/Footer.astro
  • Three columns: Product (Docs, Blog), Community (GitHub, Contributing), Legal (License)
  • "Built with Astro + Starlight" credit line
  • Copyright: "© 2025-present Bub contributors. Apache-2.0 License."
• Wire all into src/pages/index.astro
• Ensure consistent section spacing and responsive behavior

Acceptance Criteria

• Philosophy section conveys Bub's unique origin story clearly
• Architecture diagram is readable at all breakpoints
• Footer contains all required links and renders correctly
• All sections have proper dark/light mode styling
• No horizontal scroll on any viewport width

[100gle]

✅ Completed — Philosophy.astro (two-column layout: storytelling copy + pull quote + design principles highlights) and Footer.astro (brand, nav links, copyright, built-with) implemented and wired into both EN and zh-cn landing pages. i18n copy fully translated.