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Export copies the library to the clipboard as base64 JSON. Import reads the same format: paste an export, and videos not already here are added, tags on existing ones updated, nothing removed. Open the base64 library in CyberChef.
Storage is localStorage in this browser profile on this device, plus thumbnails as blobs in IndexedDB. Other browsers, other profiles, private windows and cleared site data do not share it. Quota is about 5 MB, enough for thousands of videos. Manually export/import, or use MantleDB sync, to move saved videos between browsers.
MantleDB is a free hosted JSON key-value store with no accounts: claim a namespace, receive a key, read and write over HTTPS with it. It keeps the library client-side: no backend, no sign-in, no central store. The browser talks to MantleDB directly, and only once sync is on.
Sync stores one entry: the library as a JSON array of ID, title, channel, save date and tags. Thumbnails, the current video and view settings stay local. Reads and writes need the key. Open the base64 library in CyberChef.
Limits: 64 KB per entry, roughly a few hundred videos. Namespaces are deleted after 90 days without use. No uptime or durability guarantee, so this browser holds the master copy and MantleDB is a mirror that can never cause data loss here: if the entry is gone, locked, unreadable or empty, sync turns itself off and the library stays in this browser; if it would remove or change more than a fifth of the library here, you are asked first. Last save wins; there is no merging between devices. While sync is on, MantleDB sees the client IP address on each visit. The key cannot be recovered.
Turn on sync claims the generated namespace, stores the key in this browser and pushes the library. Every page load and every change pulls first, then every save pushes. A link and QR code carry the namespace and key: open either in another browser to connect it to the same library. The link is read once, stored in that browser and removed from the address bar. Entering an existing namespace and key connects to it instead, and the videos and tags here are merged into that library.
Synced with MantleDB. Open the link or scan the code in another browser to connect it to the same library. The link carries the namespace and key: anyone with it can read and change the library, and it is the only way to reconnect after site data is cleared, so keep a copy. Disconnect removes the key from this browser and keeps the library. Open the base64 library in CyberChef.