gui/mod-manager: warn when a preset is missing new vanilla mods

[Alistair-Afton]

Fixes DFHack/dfhack#5839

Summary

When Bay12 ships a new vanilla mod, existing mod presets silently exclude it since their modlist predates it.

Presets now record the set of installed vanilla mod ids (vanilla_mods) when saved or overwritten. On load, the current vanilla set from base_available is compared against the recorded set and any new vanilla mod not in the preset's modlist is listed in the warning dialog. The recorded set is refreshed on every load so the warning fires only once per new mod, and the newly recorded ids include intentionally excluded mods, which prevents repeat false positives.

Presets saved before this change have no recorded set and record the current one silently on first load — intentionally excluded vanilla mods cannot be distinguished from missing ones at that point, and a false warning on every old preset seemed worse.

Testing

Verified live against a synthetic viewscreen_new_regionst with populated base_available vectors (three vanilla mods + one local mod), a shadowed get_any_moddable_viewscreen, and a test preset whose recorded baseline lacked one vanilla mod:

• First load: warning dialog shown listing the missing vanilla mod; the preset's mods loaded into object_load_order as before.
• The preset's recorded baseline updated to the current vanilla set.
• Second load: no warning.
• get_vanilla_mod_ids returns only data/vanilla entries.