Move the age gate to a real settings page

A date of birth had no business sitting in the content-filter popover,
next to "hide harem". It was reached through a button labelled Filters,
it vanished entirely on the My library tab, and it was per-browse
furniture rather than the account state it actually is.

Settings is now its own view, reached from the account controls in the
header rather than the browse nav, with Account, Your library and Date
of birth sections. It renders before the catalog gates, because none of
it depends on the catalog.

More importantly, it no longer advertises what it is for. The point of
the gate is that a reader who has not set a date of birth should never
learn from the app that adult content exists in it at all — not from the
browse surfaces, and not from a "confirm your age to unlock..." prompt
either. So the date of birth section says only what the date is used
for, the Adult content section renders solely for a confirmed adult, and
a reader under 18 sees no trace of it rather than an explanation of what
they are missing. The confirm and remove toasts were reworded for the
same reason.

Importing a backup is disabled until the catalog has loaded. An import
matched against an empty catalog skips work-id re-homing, which would
strand progress under ids the current catalog no longer mints.