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>

prosekit-editor.js

@@ -137,7 +137,9 @@ function defineGiteaRepoEmbed() {
           { href: url, style: 'font-weight:600;text-decoration:none;color:inherit;' },
           `${owner}/${repo}`,
         ],
-        ['div', { style: 'opacity:0.7;font-size:0.9rem;margin-top:0.25em;' }, description],
+        // Solid mid-gray, not opacity - dim on both the newsletter's
+        // light client background and this app's dark paper.
+        ['div', { style: 'color:#9a9a9a;font-size:0.9rem;margin-top:0.25em;' }, description],
       ]
     },
   })