fix(AppShell): wire up <filefiltername> hint, remove dead <preview/>

[alexwarren]

Summary

• Part of Editor: several v5 .aslx control hints not wired through to AppShell (freetext, min/max/increment, nullable, width, ...) #2116, covering two related tags on "file" pickers.

<filefiltername> (13 uses, e.g. "Picture Files" on cover art/background image pickers) was fully dead. HTML file inputs have no native filter-name UI the way v5's WPF file-open dialog did, so this is surfaced as the upload button's tooltip instead (e.g. "Upload a new file (Picture Files)"). Threaded through both the property panel (ToControlInfo/PropertyEditor.svelte) and script editor (BuildScriptControlData/ScriptEditor.svelte) file/simpleeditor="file" pickers into a new filterName prop on AssetPicker.svelte.

<preview/> (8 uses) turns out to already be fully redundant, not just dead. Every single usage pairs it with an image-only <source> filter ([EditorImageFormats] or an explicit *.jpg;*.png;... list), and AssetPicker.svelte already auto-shows a thumbnail for any picker whose extension filter is all-image (parseAssetSource's kind === "image", AssetPicker.svelte:80-88) - completely independent of this tag, which is never read anywhere in EditorCore/WasmEditorBridge.cs. Rather than wire up a no-op, removed the now-pointless tag from the 4 .aslx files it appeared in (CoreEditorGame.aslx x3, CoreEditorObjectRoom.aslx, CoreEditorScriptsOutput.aslx x2, GamebookCoreEditor.aslx x2), and confirmed by hand that the thumbnail preview still works identically without it.

Test plan

• dotnet build --configuration Release
• dotnet test --configuration Release (all passing)
• npm run check / npm run lint in src/AppShell (clean)
• node tests/e2e/find-affected-tests.mjs + ran the relevant flagged scripts (verify-appshell-library-editor, verify-appshell-library-xml-extension, verify-preview-custom-library, verify-appshell-gamebook-mode) against a local dev server - all pass.
• Manual check in-browser: the game's Cover art upload button now tooltips "Upload a new file (Picture Files)". Uploaded a synthetic red PNG as the cover art and confirmed the thumbnail preview still renders correctly next to the filename, proving the removal of <preview/> didn't regress anything - the thumbnail was never driven by that tag in the first place.

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