[OCISDEV-1087] newFileMenu extension icon is ignored unless extension/mimeType is also set

## Summary

`ApplicationFileExtension.icon` (`web/packages/web-pkg/src/apps/types.ts`) is silently ignored by the "New file" menu (`CreateAndUpload.vue`) unless the same extension entry also declares an `extension` or `mimeType`.

## Why

The "New file" dropdown renders each `newFileMenu` item's icon via:

```

```

(`web/packages/web-app-files/src/components/AppBar/CreateAndUpload.vue`, `getIconResource` builds `{ type: 'file', extension: fileAction.ext }`)

`ResourceIcon.vue` (`web/packages/web-pkg/src/components/FilesList/ResourceIcon.vue`) resolves the icon purely from a mimeType/extension lookup table (`createDefaultFileIconMapping()` plus an injected `ResourceIconMapping`) — it never reads `ApplicationFileExtension.icon` directly. That mapping is only populated for an extension entry in `announceApplicationsReady` (`web/packages/web-runtime/src/container/bootstrap.ts`) when `fileExtensions.extension` or `fileExtensions.mimeType` is set.

So an app that registers a `newFileMenu` action *without* a real file extension (e.g. because the action opens a modal instead of creating a file of one fixed type) has no way to show a custom icon in this menu — `icon` is simply dead code for this code path, even though the type declares it and other consumers (`useFileActions.ts`'s `editorActions`, via `icon: fileExtension.icon || appInfo.icon`) do read it.

`draw-io` is the only existing extension using `newFileMenu` and it happens to work only because it also sets `extension: 'drawio'`.

## Suggested fix

Have `getIconResource` / `ResourceIcon` (or a new prop) prefer an explicit `fileAction.icon` when present, before falling back to the extension/mimeType lookup — mirroring how `useFileActions.ts`'s `editorActions` already does `icon: fileExtension.icon || appInfo.icon`.

## Repro

Register a `newFileMenu` extension with `icon: 'sparkling-2'` and no `extension`/`mimeType` field (see [owncloud/web-extensions#529](https://github.com/owncloud/web-extensions/issues/529), package `web-app-ai-quick-draft-creator`). The "New" dropdown always shows the generic blank-file icon regardless of the `icon` value.

Start with ApplicationFileExtension in web/packages/web-pkg/src/apps/types.ts and trace getIconResource in web/packages/web-app-files/src/components/AppBar/CreateAndUpload.vue. Read ResourceIcon.vue and announceApplicationsReady in web/packages/web-runtime/src/container/bootstrap.ts to understand the current lookup path, then verify that a newFileMenu action with only icon: 'sparkling-2' displays that icon while existing extension or mimeType fallback behavior remains intact.

Written by the indexing model from the issue text.