fix: exclude app_theme resources from workspace tab

Theme resources (app_theme) were showing on the workspace tab alongside
regular resources. Now they are excluded from the workspace tab
(like cache and state) and the theme tab loads only app_theme resources.

Also includes review fixes:
- Remove bidirectional $effect sync in RawAppSidebar
- Avoid spreading new files object on every keystroke in FilesetEditor
- Simplify ?? undefined no-op
- Add backend validation for is_fileset + format_extension conflict
- Fix fileset alert title

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

frontend/src/routes/(root)/(logged)/resources/+page.svelte

@@ -80,6 +80,7 @@
 
 	let cacheResources: ResourceW[] | undefined = $state()
 	let stateResources: ResourceW[] | undefined = $state()
+	let themeResources: ResourceW[] | undefined = $state()
 	let resources: ResourceW[] | undefined = $state()
 	let resourceTypes: ResourceTypeW[] | undefined = $state()
 
@@ -149,7 +150,7 @@
 	let filters = useUrlSyncedFilterInstance(untrack(() => resourcesFilterSchema))
 
 	async function loadResources(): Promise<void> {
-		resources = await loadResourceInternal(undefined, 'cache,state')
+		resources = await loadResourceInternal(undefined, 'cache,state,app_theme')
 		loading.resources = false
 	}
 
@@ -163,6 +164,11 @@
 		loading.resources = false
 	}
 
+	async function loadTheme(): Promise<void> {
+		themeResources = await loadResourceInternal('app_theme', undefined)
+		loading.resources = false
+	}
+
 	async function loadResourceInternal(
 		resourceType: string | undefined,
 		resourceTypeExclude: string | undefined
@@ -490,7 +496,13 @@
 
 	// Current resources based on tab
 	let currentResources = $derived(
-		tab == 'cache' ? cacheResources : tab == 'states' ? stateResources : resources
+		tab == 'cache'
+			? cacheResources
+			: tab == 'states'
+				? stateResources
+				: tab == 'theme'
+					? themeResources
+					: resources
 	)
 
 	// Filter resources client-side for user folder filtering (admin feature)
@@ -512,8 +524,10 @@
 		filters.val
 		if ($workspaceStore) {
 			untrack(() => {
-				if (tab === 'workspace' || tab === 'theme') {
+				if (tab === 'workspace') {
 					loadResources()
+				} else if (tab === 'theme') {
+					loadTheme()
 				} else if (tab === 'cache') {
 					loadCache()
 				} else if (tab === 'states') {