Add a template-backed new-file button to the file list

The file list's "+ New" needs a typed name and seeds only '# <name>',
so every new chapter started from a blank header. Add a second button,
"+ New from template", that takes no name: it picks the first free
untitled-N.md (reusing a gap left by a deleted file), seeds it from a
configurable template, appends it to the manuscript order and selects it.

The template lives in config.json and is edited under Settings > New-file
template..., alongside the existing LanguageTool and Mistral dialogs.
Three placeholders expand at creation time: {{name}} (file stem),
{{marker}} (the configured draft marker, so a template keeps working if
the marker changes) and {{date}} (today's UTC date). Blanking the box
falls back to the built-in default, which seeds the header fields the app
already understands followed by the draft marker.

The date is computed with a local civil-from-days conversion rather than
a date crate, keeping the binary self-contained - ldd still shows only
libc/libgcc/libm.

"+ New" is unchanged; both paths now share insert_new_file. Covered by
13 new tests: placeholder expansion, untitled-N gap reuse, civil date
conversion against known dates, config.json written before this field
still deserializing (Config::load falls back to Default on a parse error,
which would otherwise discard the user's workspace and API keys), and a
round trip proving the seeded header parses back into Title/Slug/POV/goal
via the app's own preprocess::parse.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GBWj9TphFMCoh7VHaSRnvQ

src/app/ui.rs

@@ -273,6 +273,10 @@ impl App {
                         ui.close_menu();
                         self.show_mistral_settings = true;
                     }
+                    if ui.button("New-file template…").clicked() {
+                        ui.close_menu();
+                        self.show_template_settings = true;
+                    }
                 });
                 ui.menu_button("Help", |ui| {
                     if ui.button("📝 Markdown cheatsheet").clicked() {