feat(fs): hardlink import/export assets when possible, harden sync init

Importer (gamelist/launchbox file:// flows) and exporters (gamelist.xml,
metadata.pegasus.txt local exports) now hardlink media assets when source
and destination share a filesystem, falling back transparently to a copy
on EXDEV / EPERM / EOPNOTSUPP / EMLINK / EACCES (cross-device, FAT32,
exFAT, network mounts, etc.). Saves disk space and is effectively
instantaneous on large files (videos, manuals, miximages).

Covers keep a real copy (allow_link=False) because _store_cover resizes
the small cover in place via PIL.Image.save, which would truncate the
shared inode and corrupt the user's source image.

Also makes FSSyncHandler tolerate a missing/unwritable /romm/sync at
startup: an OSError from mkdir now logs a warning instead of crashing
the whole app at module-import time. Sync calls still fail at use time
if the mount remains broken — the right place to surface the error.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/handler/filesystem/sync_handler.py

@@ -12,7 +12,7 @@ class FSSyncHandler(FSHandler):
     """Filesystem handler for sync folder operations (File Transfer mode)."""
 
     def __init__(self) -> None:
-        super().__init__(base_path=SYNC_BASE_PATH)
+        super().__init__(base_path=SYNC_BASE_PATH, tolerate_missing_base=True)
 
     def build_incoming_path(
         self, device_id: str, platform_slug: str | None = None