refactor: split Rust backend into sub-modules — git, theme, github, frontmatter, commands

- git.rs (1907 lines) → 7 files: mod, history, status, commit, remote, conflict, pulse
- theme.rs (1075 lines) → 4 files: mod, defaults, seed, create
- github.rs (886 lines) → 4 files: mod, api, auth, clone
- frontmatter.rs (827 lines) → 3 files: mod, yaml, ops
- lib.rs (772 lines) → lib.rs (160) + commands.rs (650)

484 tests pass, clippy clean, rustfmt clean, coverage 85.42%

src-tauri/src/git/commit.rs

@@ -0,0 +1,87 @@
+use std::path::Path;
+use std::process::Command;
+
+/// Commit all changes with a message.
+pub fn git_commit(vault_path: &str, message: &str) -> Result<String, String> {
+    let vault = Path::new(vault_path);
+
+    // Stage all changes
+    let add = Command::new("git")
+        .args(["add", "-A"])
+        .current_dir(vault)
+        .output()
+        .map_err(|e| format!("Failed to run git add: {}", e))?;
+
+    if !add.status.success() {
+        let stderr = String::from_utf8_lossy(&add.stderr);
+        return Err(format!("git add failed: {}", stderr));
+    }
+
+    // Commit
+    let commit = Command::new("git")
+        .args(["commit", "-m", message])
+        .current_dir(vault)
+        .output()
+        .map_err(|e| format!("Failed to run git commit: {}", e))?;
+
+    if !commit.status.success() {
+        let stderr = String::from_utf8_lossy(&commit.stderr);
+        let stdout = String::from_utf8_lossy(&commit.stdout);
+        // git writes "nothing to commit" to stdout, not stderr
+        let detail = if stderr.trim().is_empty() {
+            stdout
+        } else {
+            stderr
+        };
+        return Err(format!("git commit failed: {}", detail.trim()));
+    }
+
+    Ok(String::from_utf8_lossy(&commit.stdout).to_string())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::git::tests::setup_git_repo;
+    use std::fs;
+    use std::process::Command;
+
+    #[test]
+    fn test_git_commit() {
+        let dir = setup_git_repo();
+        let vault = dir.path();
+
+        fs::write(vault.join("commit-test.md"), "# Test\n").unwrap();
+
+        let result = git_commit(vault.to_str().unwrap(), "Test commit");
+        assert!(result.is_ok());
+
+        // Verify the commit exists
+        let log = Command::new("git")
+            .args(["log", "--oneline", "-1"])
+            .current_dir(vault)
+            .output()
+            .unwrap();
+        let log_str = String::from_utf8_lossy(&log.stdout);
+        assert!(log_str.contains("Test commit"));
+    }
+
+    #[test]
+    fn test_commit_nothing_to_commit_returns_error() {
+        let dir = setup_git_repo();
+        let vault = dir.path();
+        let vp = vault.to_str().unwrap();
+
+        // Create and commit, so working tree is clean
+        fs::write(vault.join("clean.md"), "# Clean\n").unwrap();
+        git_commit(vp, "initial").unwrap();
+
+        // Committing again with no changes should fail
+        let result = git_commit(vp, "nothing here");
+        assert!(result.is_err(), "Commit should fail when nothing to commit");
+        assert!(
+            result.unwrap_err().contains("nothing to commit"),
+            "Error should mention 'nothing to commit'"
+        );
+    }
+}