fix(linux): create default Secret Service collection when missing

[juacker]

Problem

On Xubuntu live sessions, autologin setups, and fresh user accounts, creating the first provider fails with:

Failed to store provider credential: Couldn't access platform secure storage: Secret Service: no result found

gnome-keyring is running with the secrets component, but no keyring collection exists (~/.local/share/keyrings/ is empty): PAM only creates and unlocks the login collection during a password login, which never happens with autologin/live sessions. The Secret Service then has nothing behind the default alias and returns no result found, blocking onboarding.

The keyring crate (3.x) cannot self-heal: its create path for the special default target calls get_default_collection() again instead of creating a collection, so every write keeps failing.

Fix

On Linux, when a secret write fails with NoStorageAccess: ... no result found:

1. Call SecretService::create_collection("Default keyring", "default") via dbus-secret-service — the daemon shows its own password dialog (same UX as secret-tool / Seahorse), so the user owns the keyring password and the collection is the system-wide default, shared by every libsecret consumer.
2. Retry the write once.
3. If the user dismisses the dialog, return an actionable error (mentioning Seahorse and the autologin caveat) instead of the cryptic one.

dbus-secret-service is the exact crate+version keyring already uses for its sync secret-service backend, so this adds no new transitive code. Reads/deletes are unchanged (NoEntry was already handled).

Testing

• cargo check passes.
• Reproduced on a Xubuntu live ISO: provider creation failed with the error above; after creating a default collection (manually via secret-tool, which performs the same D-Bus call this PR automates) credential storage works.