add sandbox-test app content and test functions

- Add main_content.rml for sandbox-test app with test cards for Timer, JSON, Crypto, Storage
- Add sandbox test functions to shell.lua (testSandboxTimer, testSandboxJSON, etc.)
- Register sandbox-test in builtin_ids and shellNavigateTo paths
- Add sandbox-test styles to shell.rml

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

base-apps/com.mosis.sandbox-test/main_content.rml

@@ -0,0 +1,39 @@
+<!-- Sandbox Test App Content Fragment -->
+<!-- Styles are in shell.rml -->
+
+<div class="sandbox-content">
+    <div class="sandbox-header">
+        <span class="sandbox-header-title">Sandbox Test</span>
+    </div>
+
+    <div class="sandbox-body">
+        <div class="sandbox-card">
+            <div class="sandbox-card-title">Timer Test</div>
+            <div id="timer-status" class="sandbox-status">Not started</div>
+            <div class="sandbox-btn" onclick="testSandboxTimer()">Start Timer</div>
+        </div>
+
+        <div class="sandbox-card">
+            <div class="sandbox-card-title">JSON Test</div>
+            <div id="json-status" class="sandbox-status">Not tested</div>
+            <div class="sandbox-btn" onclick="testSandboxJSON()">Test JSON</div>
+        </div>
+
+        <div class="sandbox-card">
+            <div class="sandbox-card-title">Crypto Test</div>
+            <div id="crypto-status" class="sandbox-status">Not tested</div>
+            <div class="sandbox-btn" onclick="testSandboxCrypto()">Test Crypto</div>
+        </div>
+
+        <div class="sandbox-card">
+            <div class="sandbox-card-title">Storage Test</div>
+            <div id="storage-status" class="sandbox-status">Not tested</div>
+            <div class="sandbox-btn" onclick="testSandboxStorage()">Test Storage</div>
+        </div>
+
+        <div class="sandbox-card">
+            <div class="sandbox-card-title">Results</div>
+            <div id="sandbox-results" class="sandbox-results">Click buttons above to run tests</div>
+        </div>
+    </div>
+</div>