chore: bump the pinned theme to v2.7.0

v2.7.0 carries the last items of the book-theme parity gate: the pinned,
scroll-tracking "On this page" panel with nested subsections
(QuantEcon/quantecon-theme.mystmd#182), collapsible stderr warnings and the
full OpenGraph/Twitter card set (QuantEcon/quantecon-theme.mystmd#92), and a
layout correction for desktop windows 1280-1327px wide
(QuantEcon/quantecon-theme.mystmd#198). No configuration changes are needed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: mmcky

lectures/myst.yml

@@ -93,4 +93,4 @@ site:
   # Pinned theme release (see https://github.com/QuantEcon/quantecon-theme.mystmd/releases);
   # bump the vX.Y.Z in this URL to take a new theme version. The old bundled
   # QuantEcon/quantecon-theme repo is archived.
-  template: https://github.com/QuantEcon/quantecon-theme.mystmd/releases/download/v2.6.1/quantecon-theme.zip
+  template: https://github.com/QuantEcon/quantecon-theme.mystmd/releases/download/v2.7.0/quantecon-theme.zip