feat: data-cost and storage UX — footprint, sync/timestamp bytes, wifi-only (R-03/R-18/R-19)

[ChelseaKR]

A tenant on a low-end device with a metered link should never be surprised by
what habitable stores or sends. This adds three linked transparency features.

R-03 storage footprint: Vault.storage_footprint() walks the vault and returns a
StorageFootprint (sealed originals, the by-design shared-copy doubling, metadata,
total, and per-capture sizes). habitable status prints a plain "storage:" line
("… total — … sealed originals + … shared copies (originals are kept twice by
design)"), AppServer.status() surfaces the raw bytes, and the app renders them
with a one-line explanation of the doubling (EN/ES). A new human_bytes() helper
formats decimal (SI) sizes on both the Python and JS sides.

R-18 data-cost transparency: SyncResult now carries bytes_sent/bytes_received
(counted for both LocalDirTransport and the relay path); habitable sync prints
"data: sent X, received Y". Rfc3161HttpTSA records request/response byte counts,
and resolve/retimestamp print the total network bytes used. Docs note a
timestamp is typically a few KB.

R-19 Wi-Fi-only / metered options: a new additive [network] allow_metered
(default true, schema stays v1) plus --wifi-only / --allow-metered flags on
resolve, retimestamp, and sync. Because a desktop CLI cannot detect a metered
link, this is an explicit gate: when wifi-only is in effect and the operation
would hit the network (relay transport, or a real RFC 3161 authority — dev/local
authorities and directory sync are free), it refuses with a clear message. The
setting is exposed read-only in AppServer.status().

Tests in tests/test_datacost.py cover footprint math, human_bytes, sync byte
counters over LocalDirTransport, [network] parsing, and the CLI refusal/allow
paths. Docs/mobile.md gains a footprint-and-data-cost section; the persona
backlog marks R-03/R-18/R-19 done. make test/lint/type + i18n gates green.

Co-Authored-By: Claude Fable 5 noreply@anthropic.com

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Backfill PR for a completed roadmap item (roadmap/data-cost-and-storage-ux-r-03-r-18-r-19-8, 1 commit(s) over main). Part of the portfolio roadmap batch.