add sandbox support to desktop designer, fix mouse coordinates and UI issues

- Add DesktopSandbox class that integrates timer, JSON, crypto, and VirtualFS APIs
- Fix mouse coordinate handling: GLFW reports window coordinates, not physical pixels
- Fix font path resolution to search multiple locations for test apps
- Fix screenshot capture timing (capture before buffer swap)
- Fix test app CSS: use border-width instead of border:none, add display:block
- Fix test app Lua: add document nil checks, use HTML entities for symbols
- Update hot_reload to reset sandbox state on reload

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

test-apps/com.mosis.sandbox-test/app.lua

@@ -7,9 +7,12 @@ local logCounter = 0
 local function log(msg)
     logCounter = logCounter + 1
     table.insert(results, string.format("[%03d] %s", logCounter, msg))
-    local el = document:GetElementById("results")
-    if el then
-        el.inner_rml = table.concat(results, "\n")
+    -- document may not be available during initial script load
+    if document then
+        local el = document:GetElementById("results")
+        if el then
+            el.inner_rml = table.concat(results, "\n")
+        end
     end
 end
 
@@ -23,12 +26,14 @@ function goBack()
 end
 
 local function setStatus(id, status, success)
+    -- document may not be available during initial script load
+    if not document then return end
     local el = document:GetElementById(id)
     if el then
         if success then
-            el.inner_rml = "✓ " .. status
+            el.inner_rml = "✓ " .. status
         else
-            el.inner_rml = "✗ " .. status
+            el.inner_rml = "✗ " .. status
         end
     end
 end
@@ -151,8 +156,8 @@ function testStorage()
     local success = true
     local msg = ""
 
-    -- Test write
-    local writeOk = fs.write("test.txt", "Hello from sandbox!")
+    -- Test write (VirtualFS requires /data/, /cache/, /temp/, or /shared/ prefix)
+    local writeOk = fs.write("/data/test.txt", "Hello from sandbox!")
     if writeOk then
         log("Write successful")
     else
@@ -162,7 +167,7 @@ function testStorage()
 
     -- Test read
     if success then
-        local content = fs.read("test.txt")
+        local content = fs.read("/data/test.txt")
         if content == "Hello from sandbox!" then
             log("Read successful: " .. content)
         else
@@ -173,9 +178,9 @@ function testStorage()
 
     -- Test list
     if success then
-        local files = fs.list("/")
+        local files = fs.list("/data")
         if files then
-            log("Files in root: " .. #files)
+            log("Files in /data: " .. #files)
             for _, f in ipairs(files) do
                 log("  - " .. f)
             end
@@ -184,7 +189,7 @@ function testStorage()
 
     -- Test delete
     if success then
-        local deleteOk = fs.delete("test.txt")
+        local deleteOk = fs.delete("/data/test.txt")
         if deleteOk then
             log("Delete successful")
         else