fix(bridge): expose copy_from_cache_to_temp + harden launch_presentation

copy_from_cache_to_temp IPC handler was registered in file_handlers.ts
but never added to the preload bridge, making it unreachable from Svelte
despite being the documented preferred primitive for custom launch flows.

launch_presentation was the last handler still using osascript -e with
inline path injection. Converted to the temp-.scpt-file approach already
used by run_osascript and launch_from_cache — prevents breakage on
presentation filenames with spaces, quotes, or parentheses.

Also adds a pre-copy existence check to launch_from_cache so a missing
cache entry returns a meaningful error instead of a raw ENOENT.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/main/file_handlers.ts

@@ -108,6 +108,10 @@ export function registerFileHandlers() {
 
       console.log(`Native: Launching from Cache -> ${filename}`);
 
+      if (!fs.existsSync(source)) {
+        return { success: false, error: `File not in cache: ${hash}` };
+      }
+
       if (!fs.existsSync(expanded_temp)) fs.mkdirSync(expanded_temp, { recursive: true });
 
       // 1. Copy the file to temp folder with original name