fix(cli): preserve Literal type for mode in command dispatch

[Flames4fun]

Summary

This PR fixes the mypy errors reported in src/cli.py by preserving the narrow literal type used for command dispatch mode.

What changed

• Introduced a Mode type alias as Literal["fast", "smart"]
• Replaced the inline conditional assignment of mode with an explicit branch assignment
• Kept the CLI command dispatch unchanged while ensuring downstream functions receive the expected literal type

Why

The previous implementation assigned mode using a conditional expression, which caused mypy to widen the inferred type more than expected in this context. Several command handlers require Literal["fast", "smart"], so the type mismatch surfaced as multiple static type errors in src/cli.py.

Using an explicit branch preserves the intended literal typing cleanly and avoids adding unnecessary casts.

Validation

mypy src/cli.py
mypy src