No translucent input chrome: solid ink-dim placeholders and embed meta

One placeholder rule for native inputs and the prosekit editor alike;
kills ProseKit's 0.3-opacity placeholder and Firefox's UA dimming. The
embed description drops inline opacity for a fixed mid-gray that stays
dim on both the newsletter's light background and our dark paper.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

style/main.css

@@ -444,6 +444,17 @@ textarea:focus {
   box-shadow: 0 0 0 0.18rem var(--accent-soft);
 }
 
+/* One placeholder treatment for every input-like surface: solid
+   ink-dim, no translucency. opacity: 1 also undoes the UA's own
+   placeholder dimming (Firefox) and ProseKit's 0.3 on its
+   placeholder pseudo-element, which read far dimmer than the
+   native inputs next to it. */
+::placeholder,
+.prosekit-editor .prosekit-placeholder::before {
+  color: var(--ink-dim);
+  opacity: 1;
+}
+
 .prosekit-wrap {
   background: var(--paper);
   border: 0.06rem solid var(--line);