fix: suppress Rust dead code warnings with annotations

- ExportEvent::Error and DownloadEvent::Error reserved for future use
- clip_exporter functions scaffolded for caption burn-in pipeline
- SubtitleFormat fields required for serde deserialization

Co-authored-by: Cursor <cursoragent@cursor.com>

src-tauri/src/services/clip_exporter.rs

@@ -7,6 +7,7 @@ use std::process::{Command, Stdio};
 /// Strip YouTube auto-generated VTT karaoke tags and positioning metadata
 /// that confuse ffmpeg's VTT parser / mov_text conversion.
 /// Returns the path to a cleaned temp VTT file.
+#[allow(dead_code)] // Scaffolded for caption burn-in pipeline
 pub fn sanitize_vtt_for_ffmpeg(caption_path: &str, temp_dir: &str) -> Result<String, String> {
     let content = std::fs::read_to_string(caption_path)
         .map_err(|e| format!("Failed to read VTT file '{}': {e}", caption_path))?;
@@ -201,12 +202,14 @@ fn hex_to_ass_color(hex: &str) -> String {
 
 /// Convert a background opacity (0.0=transparent, 1.0=opaque) to ASS BackColour.
 /// ASS alpha: 00=opaque, FF=transparent (inverted from CSS).
+#[allow(dead_code)] // Scaffolded for caption burn-in pipeline
 fn opacity_to_ass_back_colour(opacity: f64) -> String {
     let alpha = ((1.0 - opacity.clamp(0.0, 1.0)) * 255.0).round() as u8;
     format!("&H{:02X}000000", alpha)
 }
 
 /// Build an ASS force_style string from user caption settings.
+#[allow(dead_code)] // Scaffolded for caption burn-in pipeline
 fn caption_style_to_force_style(style: &CaptionStyle) -> String {
     let font_size = (style.font_size as f64 * 2.0).round() as u32;
     let primary_colour = hex_to_ass_color(&style.text_color);