Add LanguageTool grammar/spell checking with a settings dialog

Check the current file against a LanguageTool server (typically a local
instance) from a background thread. Issues are underlined in the editor
(red spelling, blue grammar) and listed in a bottom panel with one-click
fixes that re-align the remaining matches.

A menu bar (File / View / Settings) exposes a LanguageTool settings
dialog with discrete scheme / host / port / token / language fields, an
endpoint preview, and a Test connection button. Config stores the parts
and assembles the base URL. The token is sent as a bearer header.

Uses ureq with a statically-linked rustls backend, so both http and
https endpoints work with no new runtime library dependencies.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/config.rs

@@ -24,6 +24,42 @@ pub struct Config {
     /// +100 = double, -50 = half).
     #[serde(default)]
     pub editor_zoom: f32,
+    /// URL scheme for the LanguageTool server: `http` (local) or `https`.
+    #[serde(default = "default_languagetool_scheme")]
+    pub languagetool_scheme: String,
+    /// Host of the LanguageTool server — an IP address or a domain name.
+    #[serde(default = "default_languagetool_host")]
+    pub languagetool_host: String,
+    /// TCP port of the LanguageTool server.
+    #[serde(default = "default_languagetool_port")]
+    pub languagetool_port: u16,
+    /// Optional bearer token sent in the `Authorization` header (for a server
+    /// behind an auth proxy, or the premium API key). Empty = no auth.
+    #[serde(default)]
+    pub languagetool_token: String,
+    /// Language passed to LanguageTool (`auto` to detect, or a code like `en-US`).
+    #[serde(default = "default_languagetool_language")]
+    pub languagetool_language: String,
+}
+
+/// Default scheme: plain HTTP, matching a local server.
+pub fn default_languagetool_scheme() -> String {
+    "http".to_string()
+}
+
+/// Default host: the local machine.
+pub fn default_languagetool_host() -> String {
+    "localhost".to_string()
+}
+
+/// Default port: the one the standalone LanguageTool server listens on.
+pub fn default_languagetool_port() -> u16 {
+    8010
+}
+
+/// Default checking language: let the server auto-detect.
+pub fn default_languagetool_language() -> String {
+    "auto".to_string()
 }
 
 /// Default header/body separator, matching the manuscript drafting convention.
@@ -42,6 +78,11 @@ impl Default for Config {
             draft_marker: default_marker(),
             zero_pad_index: false,
             editor_zoom: 0.0,
+            languagetool_scheme: default_languagetool_scheme(),
+            languagetool_host: default_languagetool_host(),
+            languagetool_port: default_languagetool_port(),
+            languagetool_token: String::new(),
+            languagetool_language: default_languagetool_language(),
         }
     }
 }
@@ -62,6 +103,19 @@ fn config_path() -> PathBuf {
 }
 
 impl Config {
+    /// Assemble the LanguageTool base URL (`scheme://host:port`) from its parts.
+    pub fn languagetool_base_url(&self) -> String {
+        let scheme = match self.languagetool_scheme.trim() {
+            "" => "http",
+            s => s,
+        };
+        format!(
+            "{scheme}://{}:{}",
+            self.languagetool_host.trim(),
+            self.languagetool_port
+        )
+    }
+
     pub fn load() -> Config {
         let path = config_path();
         match std::fs::read_to_string(&path) {