* fix(app): preserve composer mode preferences
Treat undefined provider preference updates as no-ops, and keep saved mode selections instead of silently defaulting them when provider metadata is incomplete or does not list the saved mode.
* fix(app): document mode preference preservation
The local/worktree choice was unstored component state and reset on every
remount of the New Workspace screen (e.g. after creating a workspace).
Persist it in the create-form preferences (FormPreferences.isolation).
Also rename the app-layer "backing" to "isolation" to match its UI label.
Isolation is the create-time intent; it stays distinct from workspaceKind,
the git-derived persisted property, and the wire's source.kind.
