fix: address folder tree QA feedback — recursive filtering, folder creation, path display, remove flatten banner

1. Folder selection now recursively includes notes from subfolders
2. + button creates a new folder with inline rename (Tauri command + UI)
3. Note list items show full path when note is in a subdirectory
4. Remove flat vault migration banner and all related code (Rust commands,
   hook, component, smoke test) — subfolders are now first-class

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/utils/noteListHelpers.test.ts

@@ -715,7 +715,12 @@ describe('filterEntries — folder selection', () => {
     expect(result.find(e => e.title === 'Site')).toBeUndefined()
   })
 
-  it('filters by parent folder (non-recursive — direct children only)', () => {
+  it('filters recursively — includes notes from subfolders', () => {
+    const result = filterEntries(entries, { kind: 'folder', path: 'projects' })
+    expect(result.map(e => e.title)).toEqual(['Note 1', 'Note 2', 'Site'])
+  })
+
+  it('filters direct children', () => {
     const result = filterEntries(entries, { kind: 'folder', path: 'areas' })
     expect(result.map(e => e.title)).toEqual(['Health'])
   })

src/utils/noteListHelpers.ts

@@ -314,12 +314,8 @@ function applySubFilter(entries: VaultEntry[], subFilter: NoteListFilter): Vault
 }
 
 function isInFolder(entryPath: string, folderRelPath: string): boolean {
-  const sep = '/'
-  const suffix = sep + folderRelPath + sep
-  const dirEnd = entryPath.lastIndexOf(sep)
-  if (dirEnd < 0) return false
-  const entryDir = entryPath.slice(0, dirEnd + 1)
-  return entryDir.endsWith(suffix)
+  const needle = '/' + folderRelPath + '/'
+  return entryPath.includes(needle) || entryPath.startsWith(folderRelPath + '/')
 }
 
 function filterByKind(entries: VaultEntry[], selection: SidebarSelection, subFilter?: NoteListFilter): VaultEntry[] {