Add transcription hook

Add support for transcription hook - an executable script in app's data directory.

If `transcription_hook` file exists, Handy runs it passing transcription text via stdin and
uses script stdout as a transcription result.

This approach is a flexible extension point for advanced users
(which nowadays means with access to coding LLM) akin to git hooks.

Here are some possible scenarios:
* simple transcription modifications
* a pipeline involving LLM processing, language detection and translation
* custom paste method (as Handy does nothing if transcription is empty)
* conditional processing based on the active application waiting for the input

See related:
* #168
* #162
* #916
* #911
* #834
* #847
* #833
* #662
* #601
* #335
* #739
* #638
* #455
* #157

Author: AlexanderYastrebov

README.md

@@ -101,6 +101,40 @@ handy --start-hidden --no-tray
 > /Applications/Handy.app/Contents/MacOS/Handy --toggle-transcription
 > ```
 
+
+### Transcription hook
+
+It is possble to modify transcription text by a script before Handy pastes it.
+To do that create an executable script `transcription_hook` in the app's data folder.
+Handy will pass transcription text to it via standard input and paste its standard output.
+
+Example script to transform first letter to lowercase and remove trailing period:
+```bash
+# macOS/Linux
+cat > ~/Library/Application\ Support/com.pais.handy/transcription_hook << 'EOF'
+#!/usr/bin/env python3
+import sys
+text = sys.stdin.read().strip()
+text = text[:-1] if text.endswith('.') else text
+text = text[0].lower() + text[1:] if text else text
+sys.stdout.write(text)
+EOF
+
+chmod +x ~/Library/Application\ Support/com.pais.handy/transcription_hook
+```
+
+Example using Apple Script to prefix transcription with `#` if active window is Terminal.app:
+```bash
+#!/bin/bash
+active_app=$(osascript -e 'tell application "System Events" to get name of first application process whose frontmost is true')
+
+if [ "$active_app" = "Terminal" ]; then
+    echo -n "# $(cat)"
+else
+    cat
+fi
+```
+
 ## Known Issues & Current Limitations
 
 This project is actively being developed and has some [known issues](https://github.com/cjpais/Handy/issues). We believe in transparency about the current state:

src-tauri/src/managers/transcription.rs

@@ -4,12 +4,14 @@ use crate::settings::{get_settings, ModelUnloadTimeout};
 use anyhow::Result;
 use log::{debug, error, info, warn};
 use serde::Serialize;
+use std::io::Write;
 use std::panic::{catch_unwind, AssertUnwindSafe};
+use std::process::{Command, Stdio};
 use std::sync::atomic::{AtomicBool, AtomicU64, Ordering};
 use std::sync::{Arc, Condvar, Mutex, MutexGuard};
 use std::thread;
 use std::time::{Duration, SystemTime};
-use tauri::{AppHandle, Emitter};
+use tauri::{AppHandle, Emitter, Manager};
 use transcribe_rs::{
     engines::{
         gigaam::GigaAMEngine,
@@ -628,7 +630,7 @@ impl TranscriptionManager {
             translation_note
         );
 
-        let final_result = filtered_result;
+        let final_result = run_transcription_hook(&self.app_handle, filtered_result);
 
         if final_result.is_empty() {
             info!("Transcription result is empty");
@@ -659,3 +661,70 @@ impl Drop for TranscriptionManager {
         }
     }
 }
+
+/// Runs a transcription hook script if it exists.
+///
+/// This function looks for an executable script named `transcription_hook`
+/// in the app's data directory, runs it passing text via stdin and
+/// return script stdout as a result.
+fn run_transcription_hook(app: &AppHandle, text: String) -> String {
+    let app_data_dir = match app.path().app_data_dir() {
+        Ok(dir) => dir,
+        Err(e) => {
+            error!("Failed to get app data directory: {}", e);
+            return text;
+        }
+    };
+
+    let transcription_hook = app_data_dir.join("transcription_hook");
+
+    if !transcription_hook.exists() {
+        debug!("Transcription hook is not configured");
+        return text;
+    }
+
+    let mut child = match Command::new(&transcription_hook)
+        .stdin(Stdio::piped())
+        .stdout(Stdio::piped())
+        .stderr(Stdio::piped())
+        .spawn()
+    {
+        Ok(child) => child,
+        Err(e) => {
+            error!("Failed to spawn transcription hook: {}", e);
+            return text;
+        }
+    };
+
+    if let Some(mut stdin) = child.stdin.take() {
+        if let Err(e) = stdin.write_all(text.as_bytes()) {
+            error!("Failed to write to transcription hook stdin: {}", e);
+            return text;
+        }
+        // stdin is dropped here closing the pipe
+    }
+
+    let output = match child.wait_with_output() {
+        Ok(output) => output,
+        Err(e) => {
+            error!("Failed to wait for transcription hook: {}", e);
+            return text;
+        }
+    };
+
+    let stdout = String::from_utf8_lossy(&output.stdout).to_string();
+    let stderr = String::from_utf8_lossy(&output.stderr).to_string();
+
+    if !output.status.success() {
+        error!(
+            "Transcription hook exited with code {:?}, stderr: {:?}",
+            output.status.code(),
+            stderr
+        );
+        return text;
+    }
+
+    debug!("Transcription hook completed, stderr: {:?}", stderr);
+
+    stdout
+}