🎨 Mosaic: UI Polish for CLI Output

[madmax983]

🖌️ Before:
CLI errors displayed raw, lowercased strings without proper punctuation (e.g., missing transaction description). This looked unpolished and like a raw log file output.

✨ After:
CLI errors are now presented as proper human-readable sentences with capitalized first letters and trailing periods (e.g., Missing transaction description.).

🖼️ Visuals:
When a user encounters a validation or parsing error, they receive a clear, grammatical sentence, which improves the overall polish and accessibility of the tool.

(Additionally updated 18 corresponding integration assertions in tests/cli_parse.rs and unit tests in src/commands/txn.rs to match the new format.)

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PR created automatically by Jules for task 5503533285705587429 started by @madmax983

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