Let the naming scheme organize files into subfolders

- rename::rename_one now resolves new_name relative to the source file's
  own directory, allowing / as a subdirectory separator (e.g. a naming
  scheme like {show name}/{episode name}) instead of rejecting it. Missing
  intermediate directories are created automatically via create_dir_all.
- New validate_relative_target() replaces the old is_plain_filename():
  still rejects anything that could escape the source directory (empty
  path segments - which also catches a leading-slash absolute path -
  and . / .. segments) plus backslashes (Renamr always treats / as the
  scheme's folder separator regardless of OS), but now allows a
  legitimate multi-segment relative path through.
- The "Unchanged" check now compares the full resolved target path to the
  source, not just the filename, since a computed name can now include
  directory components.
- 6 new tests: organizing into a new subfolder, nested subfolders that
  don't exist yet, conflict detection still works for organized targets,
  and rejection of backslashes / doubled / leading / trailing slashes.
- Frontend: naming-scheme editor hint explains the / convention, and the
  rename confirmation modal notes when a plan will create new subfolders.
- No changes needed in the naming-expression engine itself - it already
  treats / between two {} scriptlets as ordinary literal text.

README.md

@@ -9,10 +9,11 @@ native file/folder loading and drag-and-drop, local filename-based season/episod
 detection ("Match") is working, real online metadata lookups against TMDB ("Fetch
 Data") are working — including fuzzy show-name disambiguation, so a search that
 returns several similarly-named shows doesn't just take TMDB's top-ranked hit — and the
-actual on-disk rename operation is wired up (same-directory rename, skip-on-conflict,
-with a confirmation step showing the full old-name/new-name plan before anything
-touches disk — there is no undo yet). Full media-metadata bindings are not yet ported.
-See "Porting roadmap" below.
+actual on-disk rename operation is wired up (skip-on-conflict, with a confirmation step
+showing the full old-name/new-name plan before anything touches disk — there is no undo
+yet). Rename can also organize files into subfolders — including ones that don't exist
+yet — when the naming scheme includes a `/`. Full media-metadata bindings are not yet
+ported. See "Porting roadmap" below.
 
 The naming scheme is fully user-editable from the app (the "Naming Scheme" button next
 to the New Names list) — the default is `{show name} - {season number}x{episode
@@ -78,6 +79,28 @@ An undefined binding contributes an empty string rather than failing outright (s
 `{n}{missing}` still renders); the overall expression only errors if the *total*
 rendered output would be empty.
 
+`/` between two `{ }` scriptlets (or anywhere in the literal text) needs no special
+handling in the expression engine at all — it's just ordinary literal text, the same as
+a space or a dash. It only becomes meaningful one layer up, at rename time (see
+"Organizing into subfolders" below): a scheme like `{show name}/{episode name}` renders
+to a name containing a real `/`, and the rename engine treats that as "put this file in
+a subfolder" rather than rejecting it.
+
+### Organizing into subfolders
+
+A naming scheme that includes `/` (e.g. `{show name}/Season {season number}/{episode
+number} - {episode name}`) moves the file into that subfolder as part of renaming,
+creating any missing intermediate folders automatically
+(`crates/renamr-core/src/rename/mod.rs::rename_one` → `fs::create_dir_all`). This is
+still always scoped to the file's own directory tree — the target is resolved relative
+to the source file's parent directory, never as an absolute path, and every path
+component is validated (`validate_relative_target`) to reject empty segments (a
+leading, trailing, or doubled `/` — which also catches an absolute-looking
+`/etc/passwd`-style path), `.`/`..` segments, and backslashes (Renamr always uses `/`
+for the naming scheme's folder separator regardless of OS). Conflict policy is
+unchanged: if the resolved target already exists and isn't the same underlying file,
+that item is skipped rather than overwritten.
+
 ## Project layout
 
 ```
@@ -87,10 +110,11 @@ crates/renamr-core/   Pure Rust core: naming-expression engine (ported), local
                        (`similarity/`, partial `EpisodeMetrics` port), a TMDB metadata
                        client (`web/tmdb.rs` — TV search + episode lookup, disambiguated
                        via `similarity/`, tested against a mocked HTTP server via
-                       `mockito`), and the on-disk rename engine (ported for the
-                       same-directory case, `rename/`) — skip-on-conflict, no
-                       move/organize-into-folders yet. Not yet ported: full media
-                       metadata bindings, other web datasource clients (TVDB/AniDB/etc).
+                       `mockito`), and the on-disk rename engine (`rename/`) —
+                       skip-on-conflict, can also organize into (auto-created)
+                       subfolders when the naming scheme includes `/`. Not yet ported:
+                       full media metadata bindings, other web datasource clients
+                       (TVDB/AniDB/etc).
                        No Tauri dependency, so it builds and unit-tests standalone.
 src-tauri/             Tauri app shell: exposes renamr-core as commands to the frontend
                        (naming-scheme rendering, filename matching, TMDB metadata fetch,
@@ -184,13 +208,14 @@ ported from, for anyone picking this up:
    OMDb, TVmaze, OpenSubtitles) are not yet started.
 6. **Rename engine + CLI** — `net.filebot.cli.CmdlineOperations` (1235 lines) and
    `ArgumentBean` (415 lines, the full flag surface: `-rename`, `--db`, `--format`,
-   `--action`, `--conflict`, `-get-subtitles`, `-script`, etc.). The core same-directory
-   rename operation is done (`crates/renamr-core/src/rename`, fixed skip-on-conflict
-   policy, wired to the UI's Rename button behind a confirmation step). Not yet done:
-   the `--action` variants beyond rename-in-place (move/copy/symlink to a different
-   folder, which is what lets a naming scheme like `{n}/{s00e00}` organize files into
-   subfolders), the `--conflict` variants beyond skip (auto-number, override), and the
-   CLI flag surface itself.
+   `--action`, `--conflict`, `-get-subtitles`, `-script`, etc.). The rename operation is
+   done (`crates/renamr-core/src/rename`, fixed skip-on-conflict policy, wired to the
+   UI's Rename button behind a confirmation step) including the `move`-into-subfolder
+   `--action` variant — a naming scheme like `{n}/{s00e00}` organizes files into
+   subfolders, creating any missing folders automatically, always scoped to the file's
+   own directory tree. Not yet done: the `copy`/`symlink` `--action` variants, the
+   `--conflict` variants beyond skip (auto-number, override), and the CLI flag surface
+   itself.
 7. **The rest of the UI** — the Rename panel (file loading, drag-drop, live preview,
    local matching, confirmed rename) is built; the other five sidebar panels
    (Episodes/Subtitles/SFV/Filter/List) are still stub screens, plus Align up/down