feat: add shared @sexy.pivoine.art/types package and fix type safety across frontend/backend

- Create packages/types with shared TypeScript domain model interfaces (User, Video, Model, Article, Comment, Recording, etc.)
- Wire both frontend and backend packages to use @sexy.pivoine.art/types via workspace:*
- Update backend Pothos objectRef types to use shared interfaces instead of inline types
- Update frontend $lib/types.ts to re-export from shared package
- Fix all type errors introduced by more accurate nullable types (avatar/banner as string|null UUIDs, author nullable, events/device_info as object[])
- Add artist_name to comment user select in backend resolver
- Widen utility function signatures (getAssetUrl, getUserInitials, calcReadingTime) to accept null/undefined

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/frontend/src/routes/me/+page.svelte

@@ -34,7 +34,7 @@
   let lastName = $state(data.authStatus.user!.last_name);
   let artistName = $state(data.authStatus.user!.artist_name);
   let description = $state(data.authStatus.user!.description);
-  let tags = $state(data.authStatus.user!.tags);
+  let tags = $state(data.authStatus.user!.tags ?? undefined);
 
   let email = $state(data.authStatus.user!.email);
   let password = $state("");
@@ -60,8 +60,8 @@
 
       let avatarId = undefined;
 
-      if (!avatar?.id && data.authStatus.user!.avatar?.id) {
-        await removeFile(data.authStatus.user!.avatar.id);
+      if (!avatar?.id && data.authStatus.user!.avatar) {
+        await removeFile(data.authStatus.user!.avatar);
       }
 
       if (avatar?.file) {
@@ -143,10 +143,10 @@
   function setExistingAvatar() {
     if (data.authStatus.user!.avatar) {
       avatar = {
-        id: data.authStatus.user!.avatar.id,
-        url: getAssetUrl(data.authStatus.user!.avatar.id, "mini")!,
-        name: data.authStatus.user!.artist_name,
-        size: data.authStatus.user!.avatar.filesize,
+        id: data.authStatus.user!.avatar,
+        url: getAssetUrl(data.authStatus.user!.avatar, "mini")!,
+        name: data.authStatus.user!.artist_name ?? "",
+        size: 0,
       };
     } else {
       avatar = undefined;