Show NFT portfolio value using MintGarden floor prices

[nick8319]

Summary

• Adds NFT valuation to the portfolio: collection floor price (MintGarden) × owned NFT count, converted to USD via the existing XCH/USD rate
• Backend: nft_count (visible NFTs only) computed per collection via a correlated subquery and exposed on NftCollectionRecord — no migrations, no new endpoints
• Frontend: new NftPriceContext (mirrors PriceContext) with a 15-minute TTL floor cache persisted in localStorage per network, concurrency-capped MintGarden fetches (4 at a time, 10s timeout), and pure valuation math in src/lib/nftValue.ts
• Surfaces the value in four places: main "Total Estimated Balance" (+ "Includes ~$X in NFTs"), NFT collections view summary, per-collection-card floor lines, and a "Collection Floor" row on the NFT detail page
• Privacy: only public collection IDs are sent to MintGarden — never addresses or holdings; testnet uses api.testnet.mintgarden.io
• Hidden NFTs and the "Uncategorized" pseudo-collection are excluded from valuation; never-fetched/unlisted collections contribute nothing rather than blocking the UI
• Translations extracted for all four locales

Test Plan

• cargo t -p sage-wallet — 45/45 pass
• cargo clippy --workspace — clean; cargo sqlx prepare cache committed (offline build verified)
• pnpm lint (0 errors), tsc -b, pnpm build — clean
• Manual: mainnet wallet with NFT collections — totals, collection cards, NFT detail floor all render; values spot-checked against mintgarden.io
• Cold start shows tokens-only total (no layout shift); floors appear as fetched; restart shows cached values instantly
• Testnet wallet smoke test