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/config.rs

@@ -63,6 +63,27 @@ pub struct Config {
     /// Mistral API base URL (blank = the built-in default). Override for a proxy.
     #[serde(default = "default_mistral_base_url")]
     pub mistral_base_url: String,
+    /// Markdown seeded into files made with the file list's template button.
+    /// `{{name}}` expands to the file stem, `{{marker}}` to [`Config::draft_marker`]
+    /// and `{{date}}` to today's UTC date. Blank = the built-in default.
+    #[serde(default = "default_new_file_template")]
+    pub new_file_template: String,
+}
+
+/// Default new-file template: the editorial header fields this app already
+/// understands, followed by the draft marker so the body starts below it.
+pub fn default_new_file_template() -> String {
+    [
+        "# Title: {{name}}",
+        "# Slug:",
+        "# POV:",
+        "# Word Count Target:",
+        "",
+        "{{marker}}",
+        "",
+        "",
+    ]
+    .join("\n")
 }
 
 /// Default Mistral model.
@@ -132,6 +153,7 @@ impl Default for Config {
             mistral_api_key: String::new(),
             mistral_model: default_mistral_model(),
             mistral_base_url: default_mistral_base_url(),
+            new_file_template: default_new_file_template(),
         }
     }
 }
@@ -175,6 +197,16 @@ impl Config {
         }
     }
 
+    /// The new-file template to use, falling back to the built-in default when
+    /// blank, so an accidentally emptied box still produces a usable file.
+    pub fn effective_new_file_template(&self) -> String {
+        if self.new_file_template.trim().is_empty() {
+            default_new_file_template()
+        } else {
+            self.new_file_template.clone()
+        }
+    }
+
     /// The Mistral base URL to use, falling back to the built-in default when blank.
     pub fn mistral_effective_base_url(&self) -> String {
         let b = self.mistral_base_url.trim().trim_end_matches('/');
@@ -203,3 +235,61 @@ impl Config {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// A config.json written before `new_file_template` existed. `Config::load`
+    /// falls back to `Config::default()` when parsing fails, which would quietly
+    /// discard the user's workspace and API keys — so this must keep parsing.
+    #[test]
+    fn config_without_the_template_field_still_loads() {
+        let old = r####"{
+            "workspace": "/home/writer/Manuscript",
+            "export_path": "/home/writer/Manuscript/book.odt",
+            "show_preview": true,
+            "draft_marker": "### Rough Draft:",
+            "editor_zoom": 10.0,
+            "languagetool_host": "lt.example.com",
+            "mistral_api_key": "secret"
+        }"####;
+        let cfg: Config = serde_json::from_str(old).expect("old config must still parse");
+        // Pre-existing values survive.
+        assert_eq!(cfg.workspace, PathBuf::from("/home/writer/Manuscript"));
+        assert_eq!(cfg.languagetool_host, "lt.example.com");
+        assert_eq!(cfg.mistral_api_key, "secret");
+        assert!(cfg.show_preview);
+        // The new field is filled in from its default rather than left blank.
+        assert_eq!(cfg.new_file_template, default_new_file_template());
+        assert!(cfg.new_file_template.contains("{{marker}}"));
+    }
+
+    #[test]
+    fn blank_template_falls_back_to_the_default() {
+        let blank = Config {
+            new_file_template: "   \n  ".to_string(),
+            ..Config::default()
+        };
+        assert_eq!(
+            blank.effective_new_file_template(),
+            default_new_file_template()
+        );
+        let custom = Config {
+            new_file_template: "# Mine\n".to_string(),
+            ..Config::default()
+        };
+        assert_eq!(custom.effective_new_file_template(), "# Mine\n");
+    }
+
+    #[test]
+    fn config_survives_a_save_load_round_trip() {
+        let cfg = Config {
+            new_file_template: "# Title: {{name}}\n{{date}}\n".to_string(),
+            ..Config::default()
+        };
+        let text = serde_json::to_string_pretty(&cfg).expect("serialize");
+        let back: Config = serde_json::from_str(&text).expect("deserialize");
+        assert_eq!(back.new_file_template, cfg.new_file_template);
+    }
+}