Feature request: honor a user-supplied openmw.cfg in the bring-your-own-data flow (?src=local)

[dumpster-TechOps]

What would you like?

Summary

When playing with your own game files (?src=local), let the loader respect an
openmw.cfg found in (or supplied alongside) the picked folder, instead of
synthesizing the load order itself.

Current behavior

The local-folder flow ignores any openmw.cfg in the picked directory and builds
the load order heuristically: official masters in canonical order
(Morrowind.esm → Tribunal.esm → Bloodmoon.esm), then any extra mod
.esm/.esp files alphabetically. That works for "I dropped a few mods in
Data Files", but anyone with a curated desktop setup (OpenMW launcher, Mod
Organizer, mlox-sorted list) loses their load order — and load order matters for
mods that patch each other.

Proposed behavior

v1 (load order only): if the picked folder contains an openmw.cfg (or the
user provides one), use its content=, fallback-archive=, and fallback=
lines to build the load order — matching entries against the files actually
present in the picked folder. Fall back to the current alphabetical heuristic
when no cfg is found. ?nomods=1 keeps working as an escape hatch.

Later (multiple data dirs): optionally resolve multiple data= entries by
letting the user pick a parent folder (e.g. a whole OpenMW/MO2 tree) and mapping
each data= path to a subfolder by suffix match.

Constraints / notes

• A desktop openmw.cfg contains absolute native paths in data= lines
  (C:\Games\..., /home/...). Browsers cannot open arbitrary paths — the File
  System Access API only grants the directory tree the user explicitly picks —
  so data= paths must be remapped into the granted folder rather than followed
  literally. Paths outside the picked root can't be honored.
• The plumbing mostly exists: the loader already rewrites /openmw.cfg at boot;
  this changes where the content=/fallback-archive= lines come from.
• Separate from self-hosting a custom game: baked/packaged games already boot
  from their own openmw.cfg (that's how the bundled demo works). This FR is
  only about the player-side ?src=local flow.

Why it matters

Lets modded desktop setups carry over 1:1, and is a step toward treating the web
build as a real OpenMW target (game jams, total conversions) rather than a
vanilla-only player.