fix: dedup suffix format, horizon hint, JPEG export note

- #10: filename dedup counter zero-padded (_001, _002 instead of _1, _2)
- #8: hint below auto-detect button clarifies it samples first photo only
- #9: note above export button that all output is JPEG (PNG alpha lost)

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

server.py

@@ -406,7 +406,7 @@ def _run_export_job(job_id: str, req: ExportRequest):
                 base, ext = os.path.splitext(new_name)
                 candidate, n = new_name, 1
                 while candidate in used_names:
-                    candidate = f"{base}_{n}{ext}"
+                    candidate = f"{base}_{n:03d}{ext}"
                     n += 1
                 used_names.add(candidate)