feat(model-manager): add "Delete all" gallery button + clearer 401 errors

- Gallery: DELETE /api/gallery/all removes every image under output/;
  "Delete all" button with in-app confirm and a deleted/failed count.
- Downloads: surface a clear, actionable message when CivitAI/HuggingFace
  returns 401/403 (model requires login/early-access, or the key/token
  lacks access) instead of a bare error, both at resolve time and during
  the download stream.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

model-manager/app/main.py

@@ -6,6 +6,7 @@ import os
 from pathlib import Path
 from typing import Optional
 
+import httpx
 from fastapi import FastAPI, HTTPException
 from fastapi.responses import FileResponse
 from fastapi.staticfiles import StaticFiles
@@ -141,6 +142,15 @@ async def start_download(body: DownloadIn) -> dict:
 
     try:
         resolved = await registries.resolve(url)
+    except httpx.HTTPStatusError as exc:
+        code = exc.response.status_code
+        if code in (401, 403):
+            raise HTTPException(
+                400,
+                f"The source returned {code} for this model. It likely requires "
+                "being logged in / early access on that site, or your API key/token "
+                "doesn't have access to it. (Public models download fine.)")
+        raise HTTPException(400, f"Could not resolve URL (HTTP {code})")
     except Exception as exc:  # noqa: BLE001 - report resolution failure to the user
         raise HTTPException(400, f"Could not resolve URL: {exc}")
 
@@ -246,6 +256,22 @@ def list_gallery(limit: int = 60, offset: int = 0) -> dict:
             "offset": offset, "returned": len(page)}
 
 
+@app.delete("/api/gallery/all")
+def delete_all_photos() -> dict:
+    """Permanently delete every image under OUTPUT_DIR."""
+    deleted = 0
+    failed = 0
+    if OUTPUT_DIR.is_dir():
+        for path in OUTPUT_DIR.rglob("*"):
+            if path.is_file() and path.suffix.lower() in IMAGE_EXTS:
+                try:
+                    path.unlink()
+                    deleted += 1
+                except OSError:
+                    failed += 1
+    return {"deleted": deleted, "failed": failed}
+
+
 @app.get("/gallery/file")
 def gallery_file(path: str):
     try: