fix(site): mobile waitlist input height, "Demo coming soon" panel, blog cleanup

- WaitlistForm: reset the input's flex when the form stacks — the 16rem
  flex-basis was applying to the height in column layout (256px-tall input).
- DemoVideo: add a comingSoon mode that shows a big "Demo coming soon" panel
  instead of the internal PRODUCE placeholder; enable it in the hero and the
  multiplayer section.
- Blog: retitle the post to "Porting KiCad's Graphics to WebGL with Claude"
  (clean slug), set body/description to "Coming soon.", and drop the
  placeholder text from the blog index.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

site/src/content/blog/hello-world.md

@@ -1,19 +0,0 @@
----
-title: "Hello World"
-description: "First placeholder post for the KiCad WebAssembly blog."
-pubDate: 2026-06-07
----
-
-This is a placeholder blog post. Replace this body with real content.
-
-## A heading
-
-Markdown content is rendered to static HTML at build time, so this page ships
-**zero JavaScript** to the browser.
-
-- Bullet one
-- Bullet two
-- Bullet three
-
-To add another post, drop a new `.md` file in `src/content/blog/` with the same
-frontmatter fields (`title`, `description`, `pubDate`).

site/src/content/blog/porting-kicad-graphics-to-webgl-with-claude.md

@@ -0,0 +1,7 @@
+---
+title: "Porting KiCad's Graphics to WebGL with Claude"
+description: "Coming soon."
+pubDate: 2026-06-07
+---
+
+Coming soon.