chore: move unreferenced source art out of the served web assets

apps/web/public/images was shipping ~90 MB of source art (enemies, npc,
combat status icons, HUD icon originals/difficulty badges, unused
background paintings) that no component, template, or stylesheet
actually references, since Angular copies public/ verbatim into every
browser build. Moved everything not referenced under apps/web/src to a
new art/ directory at the repo root, preserving the original subfolder
layout; only the 11 files actually loaded by the app (runtime HUD/
background derivatives and their PNG fallbacks) remain under
apps/web/public/images. Documented the split in README.md.

README.md

@@ -96,6 +96,20 @@ POST /api/travel                 { "targetLocationId": "<uuid>" }
 pipe rejects any other property (e.g. a client-supplied `arrivesAt`) with
 `400 Bad Request`, since `arrivesAt` is always server-derived.
 
+## Assets
+
+`apps/web/public/images` holds only the derivative image files the Angular
+build actually serves (resized `runtime/*-128.png` HUD icons, `runtime/*-960.jpg`
+and `runtime/*-1440.jpg` background variants, and the small set of PNG
+fallbacks referenced directly by source/styles). Everything under `apps/web/public`
+is copied verbatim into the browser build, so keep that folder limited to
+files a component, template, or stylesheet actually references.
+
+Original/unresized source art (enemy, NPC, and combat-status artwork, HUD icon
+originals, difficulty badges, etc.) that isn't loaded by the app lives in
+`art/` at the repository root instead, mirroring the same subfolder layout
+(e.g. `art/enemies`, `art/npc`, `art/hud`). It is not part of any build output.
+
 ## Scripts
 
 Run these from the repository root unless noted otherwise.