feat: invoice refresh, theme picker, and agent backup workflow

[0x054]

Summary

• Add invoice refresh: recompute lines from locked entries with before/after diff in TUI (u) and CLI (ttd invoice refresh [--apply]); paid invoices allow description-only apply
• Include entry notes as bulleted line descriptions; switch markdown Work section to block layout so previews render correctly
• Add searchable theme picker modal (t / command palette) with live preview and any Textual theme saveable to config
• Require agents to run just backup before data-touching work (new rule + recipe)

Test plan

• just check and uv run pytest tests/test_invoicing/ tests/test_tui/test_app.py -k invoice
• Refresh a draft invoice after a rate change; confirm totals update on apply
• Refresh a paid invoice with description-only diff; confirm totals/set-aside unchanged
• Open theme picker (t), preview themes, save a non-default theme to config
• Markdown preview (m) shows bulleted notes on separate lines
• just backup writes timestamped .db to ~/Backups/ttd/

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