📝 docs: Update design-decisions.md with v4.x decisions

[njueeRay]

Description

The docs/design-decisions.md records why specific visual choices were made (color palette, component themes, layout decisions), but it hasn't been updated since v3.0.

Many decisions from v4.0–v4.2 are only tracked in copilot-instructions.md and meeting notes.

What to do

1. Review the "已决定的设计选择" section in .github/copilot-instructions.md
2. Review meeting notes in docs/meetings/ (especially v4 planning sessions)
3. Add missing entries to docs/design-decisions.md with:
   • Date of the decision
   • What was decided
   • Why (rationale)
   • Alternatives considered (if documented)

Decisions to add (at minimum)

• Blog content type system (4 types: insight / technical / member-essay / meeting)
• Blog filter tabs with URL hash deep linking
• CSS architecture: global.css as single source of truth
• Mobile hamburger navigation (≤768px breakpoint)
• Team page member cards showing latest blog post

Acceptance criteria

• At least 5 new decision entries added
• Each entry has date, decision, and rationale
• Existing entries preserved