Improve Configuration User Experience and Defaults

[Aurelian-Shuttleworth]

Problem

Configuring the application across different environments is inconsistent. Users must rely on a mix of command-line arguments and a brittle ~/.gcalclirc flag file. There is no centralized, declarative way to enforce defaults (e.g., military time, specific output colors) without wrapping the binary in shell aliases, which increases operational toil.

Solution

Enhance the TOML configuration loader to support a [default] section.

• Declarative Config: Specific flags (e.g., military = true, json = true) can be defined in config.toml.
• Precedence: CLI args > Config File > Defaults.
• DX: Simplifies the CLI surface area for end-users, treating configuration as structured data rather than executable flags.