[francescobianco]

First step of the dossier work for the next release (spec in #51).

What changes for the user

• New Dossier asks only for a name. The dossier is a draft, fully usable (import, ingest, ask), stored in $XDG_DATA_HOME/gknowledge/drafts/.
• Save… (header button, Ctrl+S) or closing the window gives it a folder: by default ~/Documents/Dossiers/<name> (localised Documents dir via user-dirs.dirs), or another folder from the file chooser. Existing folders are never overwritten (<name> (2)).
• Closing with a draft open asks: Save… / Keep as Draft / Cancel. Kept drafts reopen at next launch: nothing is lost.
• A saved dossier shows <name>.dossier, sources/, knowledge/, notes/; database, cache and settings live in a hidden .dossier/ with a generated .gitignore.
• <name>.dossier is registered as application/x-dossier (shared-mime-info + desktop file): double-click opens the dossier. Open Dossier… now picks a .dossier file (or a legacy gknowledge.json).
• Flatpak: --filesystem=xdg-documents/Dossiers:create so the default home works in the sandbox.

Compatibility
Projects created with 0.3 (gknowledge.json, index/, settings/ at the root) open unchanged; no migration is run. The class is still Project; the code rename and the plain-file ledger come in follow-up PRs.

Tests

• tests/test_dossier.py: drafts, saving, no-overwrite, localised Documents dir, launcher open/rename, legacy layout untouched. 44 passed with pytest; make test and make check pass.
• GTK flows driven programmatically on X11: name dialog (Create disabled until a name is typed), draft subtitle and Save button, save to the default home, close-with-draft dialog and Keep as Draft.

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