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

README.md

@@ -71,7 +71,9 @@ Terminal=false
    enclosing repository**. Accept it to have **⟳ Sync** commit and push to that
    repo; decline to leave the workspace on its own, where **Init git** creates a
    separate repository in the folder.
-3. Create files with **＋ New**, edit on the right, `Ctrl+S` (or the Save button)
+3. Create files with **＋ New** (type a name first) or **＋ New from template**
+   (no name needed — see [New files from a template](#new-files-from-a-template)),
+   edit on the right, `Ctrl+S` (or the Save button)
    to write to disk. Drag the `⠿` handles to reorder. Use the **Zoom** slider
    above the editor to scale the editor text (0% = default; **Reset** returns to
    it), and the **Contrast** slider to brighten dim editor text (0% = theme
@@ -137,6 +139,49 @@ A single number (`## Word Count Target: 1800`) sets a point goal; ranges accept
 `-`, `–`, `to`, and grouped digits (`1,500`). Like the other header lines, the
 target is stripped from the exported document.
 
+## New files from a template
+
+The file list has two create buttons:
+
+| Button | Name | Contents |
+|---|---|---|
+| **＋ New** | the name you type beside it | `# <name>` and a blank line |
+| **＋ New from template** | auto-assigned `untitled-N.md` | the configured template |
+
+**＋ New from template** takes no typed name — it picks the first free
+`untitled-N.md` (reusing a gap if you have deleted one), seeds it from the
+template, appends it to the manuscript order and selects it. Rename it later with
+the **Rename** box, or set a `# Title:` header and let the chapter title come
+from that.
+
+Edit the template under **Settings ▸ New-file template…**. The built-in default is:
+
+```
+# Title: {{name}}
+# Slug:
+# POV:
+# Word Count Target:
+
+### Rough Draft:
+
+```
+
+Three placeholders are expanded when the file is created:
+
+| Placeholder | Expands to |
+|---|---|
+| `{{name}}` | the file stem, e.g. `untitled-3` |
+| `{{marker}}` | the draft marker set in the top bar (default `### Rough Draft:`) |
+| `{{date}}` | today's date in UTC, as `YYYY-MM-DD` |
+
+Anything else in double braces is left alone. Using `{{marker}}` rather than
+typing the marker literally keeps the template working if you change the marker
+later. Emptying the box restores the built-in default, and **Reset to default**
+does the same in one click.
+
+The template is stored in `~/.config/md-manuscript/config.json`, so it is shared
+by every workspace on the machine rather than committed with a manuscript.
+
 ## Spelling (offline)
 
 Spelling is checked **live as you type**, entirely offline — no server, no
@@ -286,7 +331,7 @@ only triggers in the header region, never in the prose below the marker.
 | `<workspace>/*.md` | your manuscript files |
 | `<workspace>/order.json` | the manual ordering (committed to git) |
 | `<workspace>/titles.json` | chapter-title overrides (committed to git) |
-| `~/.config/md-manuscript/config.json` | last workspace, export path, prefs |
+| `~/.config/md-manuscript/config.json` | last workspace, export path, prefs, new-file template |
 
 ## Markdown supported in ODT export