feat: add Chats Archive as a core extension

[platberlitz]

Adds the extension I was drafting, Chats Archive, as a Core Extension.

Where Does It Live

Next to the new character icon in the character menu. It looks like an archive button.

What it does

• Lists linked character and group chats plus root-level JSONL files in one place, including filename-only entries when a linked file is malformed.
• Groups by owner, type, or folder; sorts by date, owner, name, size, or message count (full dropdown or quick pills)
• UI you can customise to: comfortable, compact, and minimal list densities.
• Filters to one character or group at a time from a searchable avatar selector that puts same named characters with their actual file names and specific character avatars to differentiate them.
• Has multiple types of filters including name, tag, size, etc.
• Has organisation features such as folders and favourites.
• Can select multiple chats and applies favorite, folder, collection, or tag changes in one batch.
• Exports and imports the organization sidecar as a backup.
• Finds deleted-character and unlinked-group chats when you choose Find orphaned files.
• Searches message content across linked chats and every orphan currently shown. Search can be stopped, and partial failures are reported with a warning.
• Inspects the original JSONL through read-only export/view routes, including root and scanned orphan files.
• Shows system/user/assistant roles, response alternatives, chat metadata, extension-data values, and the original raw file. Has short truncated previews for each file.
• Downloads the selected chat as its original JSONL or plain text file.
• Opens linked character or group chats. Orphan files cannot be opened as active chats without relinking them.
• Renders long archives and chats in batches of 100 items.

Inspection, search, organization, and download do not modify chat files. Organization data is stored separately in the user-scoped _sbca_organization.json file. That's what the organization sidecar is for.

Tested

Used on both mobile and desktop. Performance is fairly robust and UI is simple.
I have over 20,000 chats and over 300 characters, so the stress testing went well.

[TheLonelyDevil9]

Looks solid