chore: remove dead and non-functional code

[calebrob6]

Summary

Remove verified dead modules and visible admin controls that do not perform any action. This is the first focused cleanup from the repository simplification audit.

Scope

• Delete unused UI code with no repository call sites:
  • resolveVarConcatChar in ui/src/util/api.js
  • ImageLayerInfoModal.jsx
  • Home/DisasterEvents.jsx
  • HelpDocs/HelpDocsPrevNext.jsx (also calls an undefined navigate if mounted)
• Remove the New Base Model and New Source Type buttons and their duplicate modal files. Both dialogs contain uncontrolled inputs and a Submit button with no handler, validation, or API call.
• Remove the Base Model Edit/Remove and Source Type Edit/Remove menu items, which likewise have no handlers. Preserve Source Type mobile View/Hide Info behavior.
• Remove direct UI dependency d3 and ui/public/assets/geo/world.geojson; their only consumer is the deleted DisasterEvents. Keep the separate country-boundary asset used by live forms.
• Remove import-time sys.path.append(...) from hastegeo.core.models, processors, data_layer, and utils. Repository-wide AST search found no bare sibling imports that rely on them.

Acceptance criteria

• Repository-wide searches find no remaining code references to deleted modules, helper, dependency, or asset.
• Admin Base Models and Source Types retain list/sort behavior and Source Type responsive info; non-functional create/edit/remove controls are gone.
• Importing modules from all four hastegeo.core packages works without sys.path mutation.
• UI production build and all discovered UI tests pass.
• Targeted Python import/tests pass.
• No replacement abstraction is added for code that is simply dead.

Out of scope

• Implementing base-model or source-type creation/edit/removal.
• Broader lint, formatting, or UI refactors.
• Other findings from REPOSITORY_SIMPLIFICATION_AUDIT.md.