Add nested folders to the file panel and File ▸ New project

Two features that arrived together, since the second depends on the first
to show what it generates.

Nested folders
--------------
`App::files` now holds workspace-relative paths (`part-1/ch-03.md`) rather
than bare names, and the workspace scan recurses eight levels, skipping
dot-directories, `target/` and `node_modules/`. `order::tree_order`
normalises the flat order so every folder's files are contiguous and each
folder sits where its earliest-ordered file put it — which is what keeps
the panel and the export in agreement: the export concatenates the tree
read top to bottom.

The panel draws a collapsible tree. Dragging within a folder reorders as
before; dropping onto a folder header, or among another folder's files,
moves the file on disk and carries its title override, session word
baseline and cached header info with it. Emptied folders are pruned. New
files take a path (`part-1/ch-01`) to create folders, and the Rename box
now holds the whole relative path, so editing its folder part moves the
file.

File ▸ New project
------------------
Scaffolds a project from a cookiecutter template and opens its drafting
subfolder (`06-First Draft`) as the workspace. `cookiecutter.rs` resolves
the executable from PATH and the usual per-user Python prefixes — a
desktop launcher inherits neither a conda PATH nor the tools a template's
hooks shell out to, so the resolved binary's directory is prepended for
the child — builds the non-interactive command line, and identifies the
result by diffing the output directory, which works whatever a template
names its root.

Generation runs off the UI thread because hooks can reach the network.
Settings ▸ New project… covers the template path, the subfolder to open,
the cookiecutter path, a hooks toggle, and the Gitea credentials passed
to hooks as GITEA_URL / GITEA_USER / GITEA_TOKEN.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011ZGoPiDuZ7vmryNCJWjYSD

src/app/ui.rs

@@ -221,6 +221,17 @@ impl App {
         egui::TopBottomPanel::top("menubar").show(ctx, |ui| {
             egui::menu::bar(ui, |ui| {
                 ui.menu_button("File", |ui| {
+                    if ui
+                        .button("✨ New project…")
+                        .on_hover_text(
+                            "Scaffold a project from the cookiecutter template and                              open its drafting folder",
+                        )
+                        .clicked()
+                    {
+                        ui.close_menu();
+                        self.open_new_project();
+                    }
+                    ui.separator();
                     if ui.button("📂 Open workspace…").clicked() {
                         ui.close_menu();
                         self.browse_workspace();
@@ -277,6 +288,10 @@ impl App {
                         ui.close_menu();
                         self.show_template_settings = true;
                     }
+                    if ui.button("New project…").clicked() {
+                        ui.close_menu();
+                        self.show_project_settings = true;
+                    }
                 });
                 ui.menu_button("Help", |ui| {
                     if ui.button("📝 Markdown cheatsheet").clicked() {