Fix preview cutoff + sticky left panel

1. Preview endpoint now supports HTTP Range requests (HTTP 206 Partial)
   - HTML5 video player needs Range support to seek/buffer properly
   - Without it, video would cut off after a few seconds
   - Returns chunks of 64KB on demand

2. Left panel (upload form) is now sticky (position: sticky)
   - Stays in view while right panel (jobs list) scrolls
   - On mobile (<800px) reverts to normal flow

app/main.py

@@ -29,7 +29,7 @@ from fastapi import (
     BackgroundTasks, Request, status
 )
 from fastapi.responses import (
-    FileResponse, HTMLResponse, StreamingResponse, JSONResponse
+    FileResponse, HTMLResponse, StreamingResponse, JSONResponse, Response
 )
 from fastapi.staticfiles import StaticFiles
 from fastapi.security import HTTPBasic, HTTPBasicCredentials
@@ -667,14 +667,59 @@ async def download(job_id: str, user: str = Depends(check_auth)):
 
 
 @app.get("/api/preview/{job_id}")
-async def preview(job_id: str, user: str = Depends(check_auth)):
+async def preview(job_id: str, request: Request, user: str = Depends(check_auth)):
+    """Video preview z Range request podporo (potrebno za HTML5 video player)."""
     job = load_job(job_id)
     if not job or job.get("status") != "done":
         raise HTTPException(404, "Ne pripravljen")
     out = Path(job["output_path"])
     if not out.exists():
         raise HTTPException(404, "Output ne obstaja")
-    return FileResponse(out, media_type="video/mp4")
+
+    file_size = out.stat().st_size
+    range_header = request.headers.get("range") or request.headers.get("Range")
+
+    if range_header:
+        # Parse "bytes=START-END"
+        try:
+            range_str = range_header.replace("bytes=", "").strip()
+            start_s, end_s = range_str.split("-")
+            start = int(start_s) if start_s else 0
+            end = int(end_s) if end_s else file_size - 1
+            end = min(end, file_size - 1)
+            if start > end or start >= file_size:
+                return Response(status_code=416)  # Range Not Satisfiable
+            chunk_size = end - start + 1
+
+            def iter_file():
+                with open(out, "rb") as f:
+                    f.seek(start)
+                    remaining = chunk_size
+                    while remaining > 0:
+                        read_size = min(64 * 1024, remaining)
+                        data = f.read(read_size)
+                        if not data:
+                            break
+                        remaining -= len(data)
+                        yield data
+
+            headers = {
+                "Content-Range": f"bytes {start}-{end}/{file_size}",
+                "Accept-Ranges": "bytes",
+                "Content-Length": str(chunk_size),
+                "Content-Type": "video/mp4",
+            }
+            return StreamingResponse(iter_file(), status_code=206, headers=headers,
+                                      media_type="video/mp4")
+        except (ValueError, IndexError):
+            pass
+
+    # Brez Range — vrni cel file
+    return FileResponse(
+        out,
+        media_type="video/mp4",
+        headers={"Accept-Ranges": "bytes", "Content-Length": str(file_size)},
+    )
 
 
 @app.delete("/api/jobs/{job_id}")

templates/index.html

@@ -56,9 +56,20 @@
       display: grid;
       grid-template-columns: 1fr 1fr;
       gap: 24px;
+      align-items: start;
+    }
+    main > section.card:first-of-type {
+      position: sticky;
+      top: 16px;
+      max-height: calc(100vh - 32px);
+      overflow-y: auto;
     }
     @media (max-width: 800px) {
       main { grid-template-columns: 1fr; }
+      main > section.card:first-of-type {
+        position: static;
+        max-height: none;
+      }
     }
     .card {
       background: var(--panel);