feat(client): make the key backup a choice, and raise the passphrase floor

Two things the multi-browser story made obvious.

**The backup is now opt-out.** Keys are kept, encrypted with the passphrase, on
every node whose group you join — that is what lets a second browser recover
them, and it is finding C4: a PBKDF2-protected blob on other people's disks,
attackable offline at the speed of PBKDF2, which is memory-light and therefore
cheap on a GPU. Until now everybody paid that cost, including people who will
only ever use one browser and get nothing back for it.

Settings → "Use this account on other devices". Turning it off does not merely
stop future uploads: the next connection to each node withdraws what that node
already holds (new keypair_bundle_delete, which only ever deletes the caller's
own, taken from the authenticated session and never from the message). The
warning says plainly what it costs — clearing the browser then loses everything
encrypted for that account, with no recovery, which is the point of choosing it.

Default is on. Silent, unrecoverable key loss is worse for an ordinary user than
an exposure the roadmap already tracks, but that is a judgement call and it is
now visible and reversible instead of implicit.

**Passphrase floor 8 → 12 characters, plus a strength estimate** shown while
typing, with a refusal below ~60 bits. This number matters more here than in
most applications: it is what stands between a node operator and your identity
keys. It has to live in the client — with the password split (T1) the hub never
sees a password and cannot enforce anything about one — so the UI says why it
is asking, rather than nagging.

The estimator is deliberately conservative and dependency-free: character
classes and length, penalised for repetition and for the handful of patterns
everyone tries.

Verified against the live deployment: withdrawing the backup leaves a second
browser unable to recover anything, which is exactly what it promises, and
re-enabling restores it.

Tests: 338.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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