FLM: add guarded custom trial editor

[Dubbdee]

Summary

• add a complete custom FLM trial editor for generic lateral settings, friction curves, and every supported vehicle knob
• expose per-control help plus analysis rationales and keep Stock vs Current limited to values that actually changed
• gate custom values behind Custom FLM Values in Galaxy Developer settings
• enforce server-side numeric limits of +/-20% from the Current values captured by the analysis; only a new analysis resets the range
• preserve existing custom trials and saved values, with a one-time migration that leaves existing users enabled

Manual rlog upload is intentionally not included in this PR.

Safety

• retain the existing off-road requirement and exact rollback baseline
• validate the full submitted control set on the server
• allow one control step from a zero baseline so zero-valued knobs are not permanently locked
• clip fill presets to the analysis window and label them when clipping occurs
• disabling the toggle hides/blocks custom editing without deleting active or saved tune data

Validation

• custom trial and migration tests: 7 passed
• Galaxy settings layout tests: 11 passed
• full Windows regression: 66 passed; the same 3 pre-existing POSIX process/signal tests are not runnable on Windows
• JavaScript syntax, Python compilation, and diff checks passed