Add shared gx-dates key dates table style and respect markdown table alignment - #4575

Depends on #4574. This branch is built on top of it (the Abstracts and Fellowships date tables only exist there), so the diff here includes #4574's commits until it merges. Only the last commit is new.

Markdown tables on the hub silently ignored column alignment, and the GCC2027 key dates tables each carried their own <style> block to work around it. This moves that styling into the shared CSS:

• Respect markdown table column alignment (:---:, ---:) site-wide in global.css. The typography plugin forced text-align: start on every cell, overriding the align attribute the markdown renderer outputs
• Add a gx-dates class for key dates tables: wrap a 3-column markdown table (Date | status icon | What) in <div class="gx-dates"> and its status-icon column is centred and shrunk to its content. Targets the second column by position, so centring other columns behaves normally
• Document the Key Dates Table pattern in astro/DESIGN.md
• Use gx-dates on the GCC2027 Key Dates, Abstracts and Fellowships pages and remove their per-page <style> blocks

Example

Markdown	Renders as
<div class="gx-dates"> | Date | | What | | --- | --- | --- | | November 15, 2026 | 🟢 | 📝 Abstract submission opens | | April 15, 2027 | 🔴 | 🎤 Abstracts due for talks | | May 17, 2027 | 🔴 | 🖼️ Abstracts due for posters/demos | </div>

Note: since alignment is now respected site-wide, older tables that already asked for alignment (e.g. GCC2026's right-aligned register and key dates columns) will now render aligned, as their authors intended.