[Bug] Images are loaded ad hoc instead of via an image registry

[travkin79]

Summary
Images across the ui and ui.jobs bundles are loaded at each call site (mostly through
UiUtils.buildImageFromPngPath(...) / buildImageDescriptorFromPngPath(...)) instead of through a central
ImageRegistry. Each caller passes a hard-coded resource path, gets a freshly created Image, caches
it in a field, and is responsible for disposing it. This is repetitive, leak-prone, and hard to maintain.

Problems

• Manual disposal is error-prone and duplicated — every image owner repeats image instantiation, caching, disposal checks, and registers dispose listeners. Every occurrence is a chance to leak, to access a disposed image, or to double-dispose.
• Actual resource leaks — some images are created as static fields and never disposed.
• Fragile hand-rolled static caches — several widgets keep a static Image field plus a manual
  disposeStaticIcons() cleanup that must stay in sync with every read.
• Duplicate image instances — buildImageFromPngPath decodes and allocates a new Image on every call,
  so the same icon is loaded many times instead of being shared/cached.
• Theme handling duplicated and easy to get wrong — dark/light selection is re-implemented inline at
  ~17 sites, each branching on UiUtils.isDarkTheme() and picking one of two hard-coded paths; inconsistent
  file names (*_black/*_white, *_dark) make it easy to swap the arguments and show the wrong variant.
• Raw path string literals instead of typed keys — icons are referenced as strings like
  "/icons/chat/tools_disabled.png" (~130+ literals), so typos or moved/renamed resources fail only at
  runtime and there is no single inventory of shipped icons.

Expected behavior
Images should be loaded, cached, and disposed using a plug-in's image registry.