Fee-aware gift minimum; move gift entrypoint to Send page

Minimum: the flat 546-sat floor is replaced by a fee-aware minimum
(dust + one claim fee of headroom, floored at 546) via giftMinimum(),
so the recipient clears dust comfortably even if fees rise before they
claim — e.g. ~1000 sats at 5 sat/vB, ~7300 at 50. The create form shows
the live minimum and rejects below it; buildClaimTx keeps the hard
294-sat dust guard as a final backstop (claim fails rather than handing
over dust).

UI: the gift entrypoint moves from Settings to a "Gift some Bitcoin"
link on the Send page, opening the gift view as a send sub-view with a
back button.

New strings translated across all 19 locales.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

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