Extensible Weapons System

Currently both mines and front cannons are built in and fixed.
It would be great to have an extensible system where weapons:
- [ ] Are loaded on startup and thus support for custom weapons from the user's `Documents` sub-folder.
- [ ] Can be placed on a specific location on the ship
- [ ] Can be mapped to a specific key or button.
- [ ] Support different kinds of mines ( eg. explosion, teleportation, electric shock, lose last weapon)

The issue names no files, tests, or entry points. Start by locating where the built-in mines and front cannons are created in the MonoGame project, then map the existing weapon input and ship-placement paths. Done would require all four listed capabilities, including startup loading, placement, key or button mapping, and multiple mine types.

Written by the indexing model from the issue text.