Improve error when config file is missing

[DanGould]

Summary

• When no config file exists, payjoin-cli now reports which paths were searched instead of showing a confusing "missing field" error
• Tracks whether any config file (global ~/.config/payjoin-cli/config.toml or local config.toml) was actually loaded
• Both BIP78 (V1) and BIP77 (V2) error paths updated consistently

Closes payjoin#855

Test plan

• nix fmt -- --ci passes
• cargo clippy --all-targets --keep-going --all-features -- -D warnings clean
• codespell clean
• CI passes on fork

Disclosure: co-authored by Claude

[coveralls]

Pull Request Test Coverage Report for Build 22572333999

Details

• 9 of 25 (36.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.06%) to 82.425%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
payjoin-cli/src/app/config.rs	9	25	36.0%

Files with Coverage Reduction	New Missed Lines	%
payjoin-cli/src/app/config.rs	1	76.56%

Totals
Change from base Build 22572195695:	-0.06%
Covered Lines:	10632
Relevant Lines:	12899

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💛 - Coveralls

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