Logging: Make log messages consistent

Log messages (especially from `src/main.cpp`) should be checked for consistency.

- Uppercase/lowercase (as suggested by @drummer1154 in #1284)
- Trailing dots
- Indentation
- Leading characters such as `-`

The rules (e.g. leading `-` for command line flag-related output?) should be documented somewhere.

Start by reviewing the logging call sites in src/main.cpp and follow related messages across the source tree. Read the consistency discussion referenced in issue #1284, then identify the project’s agreed rules for casing, punctuation, indentation, and leading characters. Done means the affected messages follow documented rules and the rules are documented somewhere accessible.

Written by the indexing model from the issue text.