feat(web): enhanced file panel — side-by-side diff, hide whitespace, inline review

Adds DiffSplitView component for side-by-side diff mode, whitespace-only
change filtering, inline review comments with thread/gutter cell UI, diff
preferences persistence, and write-file API support for in-browser editing.

Backend: hideWhitespace param on git diff endpoint, write_file route.

apps/web/src/stores/useDiffCommentStore.ts

@@ -0,0 +1,92 @@
+import { useState, useEffect, useCallback } from 'react';
+
+export interface DiffComment {
+  id: string;
+  body: string;
+  createdAt: number;
+  updatedAt: number;
+}
+
+export interface DiffCommentTarget {
+  filePath: string;
+  side: 'old' | 'new';
+  lineNumber: number;
+}
+
+function loadFromStorage(key: string): Map<string, DiffComment[]> {
+  try {
+    const raw = localStorage.getItem(key);
+    if (!raw) return new Map();
+    const parsed = JSON.parse(raw);
+    return new Map(Object.entries(parsed));
+  } catch {
+    return new Map();
+  }
+}
+
+function saveToStorage(key: string, map: Map<string, DiffComment[]>) {
+  const obj: Record<string, DiffComment[]> = {};
+  for (const [k, v] of map) obj[k] = v;
+  localStorage.setItem(key, JSON.stringify(obj));
+}
+
+export function useDiffComments(sessionId: string, mode: string) {
+  const storageKey = `boocode.diff.comments.${sessionId}.${mode}`;
+  const [comments, setComments] = useState<Map<string, DiffComment[]>>(() =>
+    loadFromStorage(storageKey)
+  );
+
+  useEffect(() => {
+    saveToStorage(storageKey, comments);
+  }, [storageKey, comments]);
+
+  const addComment = useCallback(
+    (key: string, comment: DiffComment) => {
+      setComments((prev) => {
+        const next = new Map(prev);
+        const list = next.get(key) ?? [];
+        next.set(key, [...list, comment]);
+        return next;
+      });
+    },
+    []
+  );
+
+  const updateComment = useCallback(
+    (key: string, id: string, body: string) => {
+      setComments((prev) => {
+        const next = new Map(prev);
+        const list = next.get(key);
+        if (!list) return prev;
+        next.set(
+          key,
+          list.map((c) =>
+            c.id === id ? { ...c, body, updatedAt: Date.now() } : c
+          )
+        );
+        return next;
+      });
+    },
+    []
+  );
+
+  const deleteComment = useCallback(
+    (key: string, id: string) => {
+      setComments((prev) => {
+        const next = new Map(prev);
+        const list = next.get(key);
+        if (!list) return prev;
+        const filtered = list.filter((c) => c.id !== id);
+        if (filtered.length === 0) {
+          next.delete(key);
+        } else {
+          next.set(key, filtered);
+        }
+        return next;
+      });
+    },
+    []
+  );
+
+  return { comments, addComment, updateComment, deleteComment };
+}