fix(plugins): file events are objects, deletion is a typed error, DocumentFrame owns its fallback

- watch listeners receive { type: "changed" } | { type: "deleted" }, so later
  versions can add fields without breaking them.
- PluginFileDeletedError joins PluginFileConflictError. The read hooks report a
  deleted file with it, and a write over a deleted file rejects with it; Source
  shows that as the deleted notice. PluginFileConflictError now means only
  "changed on disk".
- DocumentFrame on Android shows Paseo's "<TYPE> files can't be shown on this
  device" message for documents its WebView cannot display, so the PDF example
  is useFileUrl + Loading + DocumentFrame with no platform branch.

plugin-examples/markdown-panel/client/rich-markdown.tsx

@@ -1,5 +1,6 @@
 import {
   PluginFileConflictError,
+  PluginFileDeletedError,
   type PluginFilePanelProps,
   useFileText,
 } from "@getpaseo/plugin/client";
@@ -47,6 +48,7 @@ function MarkdownEditor({ file, tab, theme }: PluginFilePanelProps) {
         await file.writeText(next, { force });
       } catch (cause) {
         if (cause instanceof PluginFileConflictError) setState("conflict");
+        else if (cause instanceof PluginFileDeletedError) setState("deleted");
         else setError(messageOf(cause));
         return;
       }
@@ -77,7 +79,7 @@ function MarkdownEditor({ file, tab, theme }: PluginFilePanelProps) {
   useEffect(() => {
     void load();
     const unwatch = file.watch((event) => {
-      if (event === "deleted") setState("deleted");
+      if (event.type === "deleted") setState("deleted");
       else if (unsaved.current === null) void load();
       else setState("conflict");
     });