chore: release v1.7.1

- Bump header-version in index.html to v1.7.1
- Bump manifest.json version to 1.7.1
- Update CHANGELOG with v1.7.1 release notes

Includes:
- Destructive confirm modal with 5s auto-countdown (throwAll, submitUseAll)
- Undo button visibility fix in history log
- undoTransactionEntry() uses custom modal instead of native confirm()

index.html

@@ -67,7 +67,7 @@
         <!-- Title — left-aligned; grows to fill space -->
         <div class="header-title-wrap">
             <h1 class="header-title" onclick="showPage('dashboard')">
-                <span data-i18n="nav.title">🏠 EverShelf</span><span class="header-version">v1.6.0</span>
+                <span data-i18n="nav.title">🏠 EverShelf</span><span class="header-version">v1.7.1</span>
             </h1>
             <!-- Update badge — shown alongside title, never replaces it -->
             <span class="header-update-badge" id="header-update-badge" style="display:none"></span>