Persist site history to disk and integrate with settings

Add persistent site history storage and plumbing between the renderer settings UI and the native app. The app now loads/saves site_history.txt in the user data directory (max 200 entries, http/https-only, stored one URL per line) and records visited sites on navigation. Settings pages receive the history via injected JavaScript when the settings page finishes loading, and a "clear-site-history" message from the settings UI clears the on-disk history and updates the renderer.

Other changes: allow settings-related process messages from content frames in the CEF client, introduce OnContentLoadFinished to trigger history injection, expose electronAPI.send/sendToHost (and reuse the native postMessage handler) in the V8 context, and remove the BigPicture in-app history UI/refresh/clear handlers (history is now managed by the native app). Also cleaned up includes and added helper utilities for JSON escaping, lowercasing, and file path handling. The initial tab restore logic was simplified to always create an initial tab (home or initial_url) and persist the session.

ui/pages/bigpicture.html

@@ -72,10 +72,6 @@
             <span class="material-symbols-outlined">bookmarks</span>
             <span class="nav-label">Bookmarks</span>
           </button>
-          <button class="nav-item" data-section="history" data-focusable tabindex="0">
-            <span class="material-symbols-outlined">history</span>
-            <span class="nav-label">History</span>
-          </button>
           <button class="nav-item" data-section="downloads" data-focusable tabindex="0">
             <span class="material-symbols-outlined">download</span>
             <span class="nav-label">Downloads</span>
@@ -123,13 +119,6 @@
             </div>
           </div>
 
-          <!-- Recent sites -->
-          <div class="recent-sites">
-            <h2 class="subsection-title">Continue Browsing</h2>
-            <div class="horizontal-scroll" id="recentSitesScroll">
-              <!-- Recent sites will be populated dynamically -->
-            </div>
-          </div>
         </section>
 
         <!-- Browse section (for webview) -->
@@ -158,27 +147,6 @@
           </div>
         </section>
 
-        <!-- History section -->
-        <section id="section-history" class="bp-section">
-          <div class="section-header">
-            <h1 class="section-title">History</h1>
-            <p class="section-subtitle">Recently visited sites</p>
-          </div>
-          <div class="section-actions">
-            <button class="action-btn" id="clearHistoryBtn" data-focusable tabindex="0">
-              <span class="material-symbols-outlined">delete_sweep</span>
-              <span>Clear History</span>
-            </button>
-            <button class="action-btn" id="refreshHistoryBtn" data-focusable tabindex="0">
-              <span class="material-symbols-outlined">refresh</span>
-              <span>Refresh</span>
-            </button>
-          </div>
-          <div class="list-container" id="historyList">
-            <!-- History will be populated dynamically -->
-          </div>
-        </section>
-
         <!-- Downloads section -->
         <section id="section-downloads" class="bp-section">
           <div class="section-header">