docs: billable expenses, Log screen, and JSON v2 backup

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Summary

• Add Expenses guide and generated ttd expense CLI reference
• Update invoicing, TUI, managing entries, keyboard shortcuts, import/export, data & backups, and configuration guides for v0.9.0 behavior
• Regenerate TUI screenshots (log-month.svg, quick-log chooser); seed demo expenses in sandbox-only seed_docs_demo.py

Known gaps (issues filed)

• invoice show table output should include expense lines #17 — invoice show table omits expense lines
• Include expense totals in ttd report #18 — reports ignore expenses
• Interactive ttd expense add -i with recall picker #19 — interactive ttd expense add -i
• Export/import expenses in CSV, XLSX, and Numbers #20 — tabular expense export
• TUI invoice refresh should show expense subtotal diff (CLI parity) #21 — TUI refresh expense subtotal diff

Safety

Ledger backup before screenshot work: ~/Backups/ttd/ttd-20260707-121946.db. just docs-shots uses a temp sandbox DB only.

Test plan

• just check
• prek run zensical-build
• just docs-cli (expense page + index)
• just docs-shots (log-month shows expense row)

Made with Cursor