Tasks

• src/config.rs: signature and signature-delim on AccountConfig and on the global Config, both in RENDER_ORDER; the account block's doc comment no longer calls them TUI-only, nothing per-account being so any more.
• src/account/context.rs: the two fields on the runtime Account, merged account-over-global, plus resolve_signature, which lets --signature-file win by standing the config down, and signature_delim, which defaults to DEFAULT_SIGNATURE_DELIM.
• src/shared/message/builder.rs: BuilderArgs::signature_delim, threaded into compose_body, which writes the separator verbatim instead of the hardcoded "\n\n-- \n".
• src/shared/message/{compose,reply,forward}.rs: the signature resolved through the account, the delimiter read off it, and the --signature help naming both.
• himalaya-tui: signature_block assembling the same two keys before mml sees them, making its signature-delim live; its own change, spec and log entry.
• Tests: the default block byte-identical to what was hardcoded, a custom and an empty delimiter, the --signature / --signature-file precedence, the default separator, and the TUI's block including the no-signature case.
• config.sample.toml on both sides, and the CHANGELOG entries.
• Fold the delta into cairn/spec/config.md and cairn/spec/commands.md; write cairn/log/2026-08-16-compose-signature.md.