refactor: extract inline view CSS into dedicated stylesheets

Move the hardcoded <style> blocks from the single-email view and the admin
email preview iframe into src/styles/*.css so they benefit from Prettier,
linting, and syntax highlighting like the rest of the design system.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/styles/email-preview.css

@@ -0,0 +1,26 @@
+body {
+  font-family:
+    -apple-system, BlinkMacSystemFont, "SF Pro Text", "SF Pro Display",
+    "Helvetica Neue", Arial, sans-serif;
+  line-height: 1.5;
+  padding: 16px;
+  margin: 0;
+  color: #333;
+  box-sizing: border-box;
+}
+img {
+  max-width: 100%;
+  height: auto;
+}
+a {
+  color: #0070f3;
+}
+@media (prefers-color-scheme: dark) {
+  body {
+    background-color: #1c1c1e;
+    color: #ffffff;
+  }
+  a {
+    color: #0a84ff;
+  }
+}

src/styles/entry.css

@@ -0,0 +1,43 @@
+body {
+  font-family: sans-serif;
+  max-width: 800px;
+  margin: 0 auto;
+  padding: 1rem;
+}
+.meta {
+  color: #666;
+  font-size: 0.875rem;
+  margin-bottom: 1.5rem;
+  border-bottom: 1px solid #eee;
+  padding-bottom: 0.75rem;
+}
+.meta dt {
+  display: inline;
+  font-weight: bold;
+}
+.meta dd {
+  display: inline;
+  margin: 0 1rem 0 0.25rem;
+}
+.attachments {
+  margin-top: 2rem;
+  border-top: 1px solid #eee;
+  padding-top: 1rem;
+}
+.attachments h2 {
+  font-size: 1rem;
+  margin: 0 0 0.5rem;
+}
+.attachments ul {
+  list-style: none;
+  padding: 0;
+  margin: 0;
+}
+.attachments li {
+  margin: 0.25rem 0;
+}
+.attachments .size {
+  color: #666;
+  font-size: 0.875rem;
+  margin-left: 0.5rem;
+}