Add sandbox API integration for RmlUi Lua state

- Register sandbox APIs (timer, JSON, crypto, VFS) into RmlUi's Lua state
- Add switchAppSandbox() function for context switching between apps
- Update goHome() to reset sandbox context when returning home
- Fix icon loading for third-party apps (handle full paths vs relative)
- Mirror changes between Android service (kernel.cpp) and desktop designer

This enables third-party apps to use sandbox APIs when running in RmlUi.

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

src/main/assets/apps/home/home.lua

@@ -110,10 +110,22 @@ function renderThirdPartyApps()
 
         -- Check if app has an icon
         if app.icon and app.icon ~= "" then
-            -- Third-party app icon path would be in their install directory
-            -- For now, use initial as we need file:// protocol support
-            icon_html = '<span style="font-size: 28px; color: #000000;">' .. initial .. '</span>'
+            local icon_path
+            -- Check if icon is already a full path (starts with / or contains :/)
+            if app.icon:sub(1, 1) == "/" or app.icon:find(":/") then
+                -- Already a full path
+                icon_path = app.icon
+            elseif app.install_path and app.install_path ~= "" then
+                -- Relative filename - construct full path from install_path
+                icon_path = app.install_path .. "/" .. app.icon
+            else
+                icon_path = app.icon
+            end
+            -- Use img tag for actual icon
+            icon_html = '<img src="' .. icon_path .. '" style="width: 48px; height: 48px;"/>'
+            print("[Home] Loading icon: " .. icon_path)
         else
+            -- Fallback to initial letter
             icon_html = '<span style="font-size: 28px; color: #000000;">' .. initial .. '</span>'
         end
 
@@ -151,9 +163,16 @@ function launchThirdPartyApp(package_id)
         if success then
             print("[Home] App sandbox started: " .. package_id)
 
-            -- Now load the app's UI document
+            -- Get app info for sandbox switching and UI loading
             local app_info = getAppInfo(package_id)
             if app_info and app_info.install_path and app_info.entry_point then
+                -- Switch sandbox context to this app (registers timer, fs, json, crypto APIs)
+                if switchAppSandbox then
+                    switchAppSandbox(package_id, app_info.install_path)
+                    print("[Home] Sandbox context switched to: " .. package_id)
+                end
+
+                -- Now load the app's UI document
                 local entry_path = app_info.install_path .. "/" .. app_info.entry_point
                 print("[Home] Loading app screen: " .. entry_path)
                 local loaded = loadScreen(entry_path)