chore: rename spending balance to lightning balance

[reneaaron]

Summary

• Renames "Spending Balance" → "Lightning Balance" across hub docs to match the terminology now used in Alby Hub itself (see getAlby/hub#2305).
• Renames hub/node/advanced-increase-spending-balance-with-on-chain-bitcoin.md → …-lightning-balance-…md, and updates SUMMARY.md plus the three inbound content-refs.
• Adds hub/.gitbook.yaml with a redirect from the old slug to the new one so existing external links keep working.

Casing was preserved (Spending Balance → Lightning Balance, spending balance → lightning balance). The "outbound liquidity" / "local balance" synonyms were kept where they appeared.

Notes for reviewers

• The glossary heading also moved, which means the GitBook-generated anchor …/bitcoin-wallet-glossary#spending-balance is now #lightning-balance. .gitbook.yaml redirects are page-level only, so external deep links to the old anchor will still land on the glossary page (just at the top, not at the section). Acceptable degradation.
• One pre-existing broken anchor in getting-started.md line 12 (#how-to-top-up-your-spending-balance-…) was left untouched — the heading it points to was already different before this PR.

Test plan

• GitBook preview renders the renamed page at the new URL
• All content-ref blocks pointing at the renamed page resolve
• Old URL …/hub/node/advanced-increase-spending-balance-with-on-chain-bitcoin redirects to the new one
• Sidebar entry under Node still reads "📤 Advanced: Increase Lightning Balance with on-chain bitcoin"
• No "Spending Balance" copy remains: grep -ri "spending balance" hub/ returns only the redirect entry in .gitbook.yaml

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