Add Mistral plot-beat generation (Tools menu)

New Tools ▸ "Generate plot beats (Mistral)…" command: pick a novel
proposal markdown file (characters + loose plot summary) and have the
Mistral chat API lay it out as plot beats in the classic three-act
structure. The result opens in a floating window where it can be edited,
saved as a new "<proposal> — beats.md" workspace file, or copied.

- src/mistral.rs: pure-Rust ureq/rustls POST to /v1/chat/completions
  (self-contained; no C bindings). System prompt fixes the three-act
  markdown shape; friendly API-error surfacing. Unit tests for content
  parsing, empty/blank responses, error-message extraction, and the
  missing-key guard.
- config: mistral_api_key / mistral_model / mistral_base_url with
  effective-value helpers and defaults (mistral-large-latest,
  https://api.mistral.ai).
- app: background generation + poll (editor stays responsive), a
  Settings ▸ Mistral… window (key/model/base URL), and the results
  window with save/copy.
- Docs: README "Plot beats (Mistral)" section and help cheatsheet note.

Verified: cargo build --release clean, 56 tests pass (+6 Mistral),
ldd still only libc/libgcc/libm. Clippy: no new warnings in the added
code (the two extra vs. the old 3-warning baseline are toolchain-surfaced
in pre-existing app.rs code).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XDfaEsTgcn61n3wgP62DFF

src/config.rs

@@ -53,6 +53,26 @@ pub struct Config {
     /// a [`crate::spell::DictEntry::id`].
     #[serde(default = "default_spell_language")]
     pub spell_language: String,
+    /// Mistral API key for plot-beat generation. Stored in plain text; empty
+    /// disables the feature until set.
+    #[serde(default)]
+    pub mistral_api_key: String,
+    /// Mistral model id used for plot-beat generation (blank = the built-in default).
+    #[serde(default = "default_mistral_model")]
+    pub mistral_model: String,
+    /// Mistral API base URL (blank = the built-in default). Override for a proxy.
+    #[serde(default = "default_mistral_base_url")]
+    pub mistral_base_url: String,
+}
+
+/// Default Mistral model.
+pub fn default_mistral_model() -> String {
+    crate::mistral::DEFAULT_MODEL.to_string()
+}
+
+/// Default Mistral API base URL.
+pub fn default_mistral_base_url() -> String {
+    crate::mistral::DEFAULT_BASE_URL.to_string()
 }
 
 /// Live spell checking is on by default.
@@ -109,6 +129,9 @@ impl Default for Config {
             languagetool_language: default_languagetool_language(),
             spell_check: default_spell_check(),
             spell_language: default_spell_language(),
+            mistral_api_key: String::new(),
+            mistral_model: default_mistral_model(),
+            mistral_base_url: default_mistral_base_url(),
         }
     }
 }
@@ -142,6 +165,26 @@ impl Config {
         )
     }
 
+    /// The Mistral model to use, falling back to the built-in default when blank.
+    pub fn mistral_effective_model(&self) -> String {
+        let m = self.mistral_model.trim();
+        if m.is_empty() {
+            crate::mistral::DEFAULT_MODEL.to_string()
+        } else {
+            m.to_string()
+        }
+    }
+
+    /// 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('/');
+        if b.is_empty() {
+            crate::mistral::DEFAULT_BASE_URL.to_string()
+        } else {
+            b.to_string()
+        }
+    }
+
     pub fn load() -> Config {
         let path = config_path();
         match std::fs::read_to_string(&path) {