perf: drop unused base64 return value from make_thumbnail

make_thumbnail was computing a base64 string of the thumbnail on every
image clipboard capture. The caller only ever used the raw bytes and
immediately discarded the String. Remove the base64 encode, simplify
the return type to Result<Vec<u8>>, and drop the now-unused
'use base64::Engine' import from main.rs.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

strata-daemon/src/main.rs

@@ -6,7 +6,6 @@ mod dbus_service;
 use std::sync::Arc;
 
 use anyhow::{Context, Result};
-use base64::Engine as _;
 use dbus_service::{Limits, StrataManager, SubmitRequest, THUMB_PX};
 use tokio::sync::{mpsc, Mutex};
 use tracing_subscriber::EnvFilter;
@@ -263,7 +262,7 @@ async fn process_bytes(
             tokio::task::spawn_blocking(move || make_thumbnail(&raw_for_thumb, THUMB_PX)).await?;
 
         let thumb_bytes = match thumb_result {
-            Ok((bytes, _b64)) => bytes,
+            Ok(bytes) => bytes,
             Err(e) => {
                 tracing::warn!("Image thumbnail generation failed: {e}");
                 return Ok(());
@@ -347,17 +346,13 @@ fn read_clipboard_bytes(mime: &str) -> Result<(Vec<u8>, String)> {
     Ok((buf, actual_mime))
 }
 
-/// Generate a PNG thumbnail (≤`max_px`×`max_px`) from raw image bytes.
-/// Returns `(thumbnail_bytes, base64_preview)`.
-fn make_thumbnail(raw: &[u8], max_px: u32) -> Result<(Vec<u8>, String)> {
+/// Generate a PNG thumbnail (<=`max_px`x`max_px`) from raw image bytes.
+fn make_thumbnail(raw: &[u8], max_px: u32) -> Result<Vec<u8>> {
     let img = image::load_from_memory(raw).context("Decoding image")?;
-
     let thumb = img.thumbnail(max_px, max_px);
     let mut out = Vec::new();
     thumb
         .write_to(&mut std::io::Cursor::new(&mut out), image::ImageFormat::Png)
         .context("Encoding thumbnail")?;
-
-    let b64 = base64::engine::general_purpose::STANDARD.encode(&out);
-    Ok((out, b64))
+    Ok(out)
 }