fix(ui): support unified diff patch format in session changes viewer

Session diffs now use a compact patch field instead of storing full
before/after content. Added parsePatchToBeforeAfter utility to extract
before/after from unified diff format, and updated MonacoDiffViewer to
accept patch prop as alternative to before/after strings.

packages/ui/src/lib/diff-utils.ts

@@ -2,6 +2,7 @@ const HUNK_PATTERN = /(^|\n)@@/m
 const FILE_MARKER_PATTERN = /(^|\n)(diff --git |--- |\+\+\+)/
 const BEGIN_PATCH_PATTERN = /^\*\*\* (Begin|End) Patch/
 const UPDATE_FILE_PATTERN = /^\*\*\* Update File: (.+)$/
+const HUNK_HEADER_PATTERN = /^@@ -(\d+)(?:,(\d+))? \+(\d+)(?:,(\d+))? @@/
 
 function stripCodeFence(value: string): string {
   const trimmed = value.trim()
@@ -48,3 +49,48 @@ export function isRenderableDiffText(raw?: string | null): raw is string {
   if (!normalized) return false
   return HUNK_PATTERN.test(normalized)
 }
+
+export function parsePatchToBeforeAfter(patch: string): { before: string; after: string } {
+  if (!patch || patch.trim().length === 0) {
+    return { before: "", after: "" }
+  }
+
+  const lines = patch.replace(/\r\n/g, "\n").split("\n")
+  const beforeLines: string[] = []
+  const afterLines: string[] = []
+
+  for (const line of lines) {
+    if (line.startsWith("---") || line.startsWith("+++") || line.startsWith("diff --git")) {
+      continue
+    }
+    if (HUNK_HEADER_PATTERN.test(line)) {
+      continue
+    }
+    if (line.startsWith("-") && !line.startsWith("---")) {
+      beforeLines.push(line.slice(1))
+    } else if (line.startsWith("+") && !line.startsWith("+++")) {
+      afterLines.push(line.slice(1))
+    } else if (line.startsWith(" ")) {
+      beforeLines.push(line.slice(1))
+      afterLines.push(line.slice(1))
+    } else if (line === "") {
+      beforeLines.push("")
+      afterLines.push("")
+    } else {
+      beforeLines.push(line)
+      afterLines.push(line)
+    }
+  }
+
+  while (beforeLines.length > 0 && beforeLines[beforeLines.length - 1] === "") {
+    beforeLines.pop()
+  }
+  while (afterLines.length > 0 && afterLines[afterLines.length - 1] === "") {
+    afterLines.pop()
+  }
+
+  return {
+    before: beforeLines.join("\n"),
+    after: afterLines.join("\n"),
+  }
+}