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/hooks/useDiffPreferences.ts

@@ -0,0 +1,68 @@
+import { useCallback, useEffect, useState } from 'react';
+
+export interface DiffPreferences {
+  layout: 'unified' | 'split';
+  wrapLines: boolean;
+  hideWhitespace: boolean;
+}
+
+const DEFAULT_PREFERENCES: DiffPreferences = {
+  layout: 'unified',
+  wrapLines: false,
+  hideWhitespace: false,
+};
+
+const STORAGE_KEY = 'boocode.diff.preferences';
+
+function loadPreferences(): DiffPreferences {
+  try {
+    const stored = localStorage.getItem(STORAGE_KEY);
+    if (stored) {
+      const parsed = JSON.parse(stored) as Partial<DiffPreferences>;
+      return {
+        layout: parsed.layout ?? DEFAULT_PREFERENCES.layout,
+        wrapLines: parsed.wrapLines ?? DEFAULT_PREFERENCES.wrapLines,
+        hideWhitespace: parsed.hideWhitespace ?? DEFAULT_PREFERENCES.hideWhitespace,
+      };
+    }
+  } catch {
+    // ignore parse errors
+  }
+  return DEFAULT_PREFERENCES;
+}
+
+function savePreferences(prefs: DiffPreferences): void {
+  try {
+    localStorage.setItem(STORAGE_KEY, JSON.stringify(prefs));
+  } catch {
+    // ignore storage errors
+  }
+}
+
+export function useDiffPreferences(): {
+  preferences: DiffPreferences;
+  updatePreferences: (updates: Partial<DiffPreferences>) => void;
+  resetPreferences: () => void;
+} {
+  const [preferences, setPreferences] = useState<DiffPreferences>(loadPreferences);
+
+  // Sync from localStorage on mount (handles multi-tab changes if we add a storage listener later)
+  useEffect(() => {
+    setPreferences(loadPreferences());
+  }, []);
+
+  const updatePreferences = useCallback((updates: Partial<DiffPreferences>) => {
+    setPreferences((prev) => {
+      const next = { ...prev, ...updates };
+      savePreferences(next);
+      return next;
+    });
+  }, []);
+
+  const resetPreferences = useCallback(() => {
+    setPreferences(DEFAULT_PREFERENCES);
+    savePreferences(DEFAULT_PREFERENCES);
+  }, []);
+
+  return { preferences, updatePreferences, resetPreferences };
+}