Customize Mixed Filament Names

[cmutnik]

Allow users to rename mixed filaments in the UI.

Summary

• Mixed Filaments previously had auto-generated, non-editable names ("Mixed Filament 3", "Mixed Filament 4", etc.). Users can now assign custom display names via a rename button in the sidebar.
• The custom name is a UI alias only — all slicing, tool-change, and virtual-ID logic is unchanged.
• Custom names persist across save/load via a new n<name> token in the mixed_filament_definitions project config string, with backward-compatible percent-encoding for delimiter characters.

Backward Compatibility

• Old *.3mf files load without issue; custom name defaults to empty and falls back to the auto-generated label.
• The n prefix token is new and does not conflict with any existing serialization tokens (g, w, m, d, o, u).

Reason

• This was in response to the open issue 74.
• The build from the workflows downloaded, installed, and allowed me to change the names; but I am not able to test it on my snapmaker until next week.

Demonstration

Under the "Mixed Filaments" section, clicking the pen icon will allow users to enter a custom name. This name propagates through the system but not backend code; it is essentially an alias for the UI.

[cmutnik]

Let me know if your PRs require any specific sections/info, assignees, reviewers etc.

This idea is great and I am happy to help with tickets, feature requests, or wherever I can plugin.