test: add folder tree and filtering tests; docs: ADR-0033

Add FolderTree component tests (render, expand, collapse, select,
highlight) and folder filtering tests in noteListHelpers (path
matching, sibling exclusion, archive/trash filtering).

ADR-0033 documents the decision to scan all subdirectories and
expose folder tree navigation, superseding ADR-0006's scanning
constraint.

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

docs/adr/0033-subfolder-scanning-and-folder-tree.md

@@ -0,0 +1,33 @@
+---
+type: ADR
+id: "0033"
+title: "Subfolder scanning and folder tree navigation"
+status: active
+date: 2026-03-31
+---
+
+## Context
+
+Supersedes the scanning constraint in [ADR-0006](0006-flat-vault-structure.md) which limited vault indexing to root-level `.md` files plus protected folders (`attachments/`, `assets/`).
+
+Users with folder-based workflows (PARA, Zettelkasten with folders, project directories) could not see or filter notes by directory. The vault scanner silently ignored all subdirectory `.md` files, making Laputa unsuitable for vaults with any folder structure.
+
+## Decision
+
+**Extend the Rust vault scanner to index `.md` files in all visible subdirectories, and expose the vault's folder tree via a new `list_vault_folders` Tauri command so the sidebar can render a collapsible FOLDERS section.**
+
+Hidden directories (names starting with `.`, plus `.git` and `.laputa`) are excluded from both scanning and the folder tree.
+
+## Options considered
+
+- **Option A** (chosen): Scan all subdirectories with `walkdir`, expose separate `list_vault_folders` command — simple, no schema changes to VaultEntry, folder tree is lightweight and independent of the entry cache.
+- **Option B**: Add a `folder` field to VaultEntry and derive the tree on the frontend — couples folder metadata to the entry cache, complicates cache invalidation when folders are created/deleted without file changes.
+- **Option C**: Keep flat scanning, add a "virtual folders" feature that groups by path prefix from frontmatter — doesn't solve the core problem of missing notes in subdirectories.
+
+## Consequences
+
+- All `.md` files in the vault are now indexed regardless of depth — vaults with many non-note `.md` files (e.g. node_modules) will see spurious entries. Mitigation: hidden directories are already excluded; users can add a `.laputaignore` in the future if needed.
+- The git-based cache in `cache.rs` already uses `walkdir` for change detection, so this change aligns scanning with caching.
+- `SidebarSelection` gains a new `{ kind: 'folder'; path: string }` variant — all exhaustive switches on selection kind must handle it.
+- ADR-0006's "flat vault" principle is relaxed: notes can now live in subdirectories. Type definitions still live in `type/` at the root.
+- Re-evaluate if users request recursive folder filtering (currently only direct children are shown when a folder is selected).

docs/adr/README.md

@@ -88,3 +88,4 @@ proposed → active → superseded
 | [0030](0030-rust-commands-module-split.md) | Rust commands/ module split by domain | active |
 | [0031](0031-full-app-for-note-windows.md) | Full App instance for secondary note windows | active |
 | [0032](0032-status-bar-for-git-actions.md) | Git actions (Changes, Pulse, Commit) in status bar, not sidebar | active |
+| [0033](0033-subfolder-scanning-and-folder-tree.md) | Subfolder scanning and folder tree navigation | active |