feat: add markdown rendering support for Content component

Detect markdown-formatted posts (from bots etc.) and render them
with react-markdown instead of plain text, with feed-friendly
styles (flattened headings, compact lists, scrollable tables).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/lib/markdown.ts

@@ -0,0 +1,43 @@
+/**
+ * Detects whether a string contains meaningful Markdown formatting.
+ * Strips URLs and nostr: references first to avoid false positives.
+ */
+export function containsMarkdown(content: string): boolean {
+  // Remove URLs and nostr: references to avoid false positives
+  const cleaned = content
+    .replace(/https?:\/\/[^\s)>\]]+/g, '')
+    .replace(/nostr:[a-z0-9]+/g, '')
+
+  // Strong signals — any single one triggers markdown
+  const strongPatterns = [
+    /^```/m, // code fence
+    /\|[\s]*:?-+:?[\s]*\|/ // table separator |---|
+  ]
+
+  for (const pattern of strongPatterns) {
+    if (pattern.test(cleaned)) return true
+  }
+
+  // Medium signals — need 2+ different types
+  const mediumPatterns = [
+    /^#{1,6}\s+\S/m, // ATX heading (# text), not #hashtag
+    /\*\*[^*\n]+\*\*/, // bold **text**
+    /__[^_\n]+__/, // bold __text__
+    /\[[^\]]+\]\([^)]+\)/, // link [text](url)
+    /^>\s+\S/m, // blockquote > text
+    /^[-*]\s+\S/m, // unordered list - item or * item
+    /^\d+\.\s+\S/m, // ordered list 1. item
+    /^---$/m, // horizontal rule
+    /~~[^~\n]+~~/ // strikethrough ~~text~~
+  ]
+
+  let matchCount = 0
+  for (const pattern of mediumPatterns) {
+    if (pattern.test(cleaned)) {
+      matchCount++
+      if (matchCount >= 2) return true
+    }
+  }
+
+  return false
+}