config: drop go-toml and the legacy TOML migrations

The one-shot config.toml -> config.ini and books.toml -> books.ini
migrations (a transition aid from the pre-INI format) are removed, along
with the github.com/pelletier/go-toml/v2 dependency and the now-dead
`toml:` struct tags. The live config format has been INI for many
releases; anyone still on a .toml recreates it (the format is
self-documenting on first run). Leaves 3 direct deps, all in active use
(bubbletea, lipgloss, go-pdf/fpdf). Tests updated to write INI.

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