Site redesign to match datatalks.club + declutter (mostly done)

[alexeygrigorev]

Make podwiki look like part of the main site.

• Header mirrors the main site; theme toggle reusing the shared jtd-theme localStorage key; full light+dark palette (Bootstrap-4 blue links, green headings/brand, green buttons, grey borders). DONE.
• Homepage decluttered: removed the 11-button wall and five duplicated 275-card wiki dumps. DONE.
• Flat nav, no dropdowns. DONE.