docs: replace README subtitle text with image (light/dark)

Swaps the 'Your trips. Your plan. Your server.' H3 for a rendered
subtitle image using <picture> + prefers-color-scheme, matching the
logo pattern.

README.md

@@ -6,7 +6,11 @@
   <img src="docs/logo-trek-dark.gif" alt="TREK" height="96" />
 </picture>
 
-### Your trips. Your plan. Your server.
+<picture>
+  <source media="(prefers-color-scheme: dark)" srcset="docs/subtitle-light.png" />
+  <source media="(prefers-color-scheme: light)" srcset="docs/subtitle-dark.png" />
+  <img src="docs/subtitle-dark.png" alt="Your trips. Your plan. Your server." height="28" />
+</picture>
 
 A self-hosted, real-time collaborative travel planner — with maps, budgets, packing lists, a journal, and AI built in.