fix: add error handling for missing input files

Display human-readable errors when the markdown file, photo, or
signature cannot be found instead of failing with a stack trace.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

main.py

@@ -15,7 +15,16 @@ def generate(
     ),
 ) -> None:
     """Generate a PDF CV from a Markdown file with YAML frontmatter."""
-    result = generate_pdf(input_file, output_file)
+    if not input_file.exists():
+        typer.echo(f"Error: Markdown file not found: {input_file}", err=True)
+        raise typer.Exit(1)
+
+    try:
+        result = generate_pdf(input_file, output_file)
+    except FileNotFoundError as e:
+        typer.echo(f"Error: {e}", err=True)
+        raise typer.Exit(1)
+
     typer.echo(f"Generated {result}")