Fix rendering race conditions where character sprites, pre-animations,
and paired character assets were displayed before being downloaded.
All assets referenced in an MS packet are now resolved and preloaded
into the browser cache before the animation timeline starts.

- Add unified assetCache module with session-wide promise caching
- Add preloadMessageAssets orchestrator for parallel asset resolution
- Cache fileExists HEAD requests so missing files aren't re-probed
- Preload all SFX (emote, shout, realization, stab) alongside sprites
- Use synchronous setEmoteFromUrl at all render transition points
- Graceful fallback to legacy setEmote if preloading times out

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

The whole codebase is formatted with 2 space indentation so
update .eslint.js to reflect that.

This commit includes another pass of refactoring to webAO, but
could not be tested because it has become virtually impossible
to compile GoldenLayout correctly and have it be detected as a
module by webpack. GoldenLayout has also been dormant for an
alarming amount of time, and its new maintainers have not been
doing a stellar job releasing bug fixes.

So, I give up trying to build webAO.

It's a heaping mess written in vanilla HTML/CSS/JS without the
use of a proper web framework, dependent on a raw TCP protocol
designed 7 years ago, only to be wrapped with yet another
protocol that only half the servers *want* to support, because
webAO is so bad that only incompetent players would want to keep
using it. (Unless you are a Chromebook player, in which case I am
sorry for hurling insults at you.)

webAO started off as an experiment by sD, but became real only
because I made it real. I was the one who added support for
it for tsuserver; I was the one who made the UI vaguely acceptable;
I was the one who added a button for it on the home page. It's
really my fault that this abomination lives on. I should have
learned a legitimate web framework and rewritten it when I
had the chance.