docs: Say what the recovery card is #149

The recovery card never says what it is. An heir or anyone else who finds one sees a threshold, an identifier, boxes of characters and restore steps, but nothing tells them it backs up a Bitcoin wallet or which software restores it. Step 4 only says "Restore the wallet using its documented Bitcoin Core recovery workflow", and Bitcoin Core has no codex32 restore of its own (see #88).

Add one plain line at the top of the card, for example:

This is a Bitcoin wallet backup. Restore it with Bitcoin Core and python-codex32, or with CipherStick.

Scope:

• docs/user/recovery-card.html on bails-v1-pin, which gui: Prepare records and restore from their date #113 moves to src/codex32_gui/forms/recovery-card.html.
• The 256-bit card template that docs: Size recovery cards to the backup length #96 adds.
• Keep it to one line so the card still fits one page.

Not covered elsewhere: #88 and #96 rework the card's layout and steps, and #90/#97 answer first-time questions in the guide, but none adds this line.

Context: Bails' inheritance advice currently lives in an old dialog that predates codex32. A card that names itself is the piece heirs actually hold.