Fix TS compiler warnings

[melange396]

Fix the 3 existing typescript compiler warnings in this codebase, as can be seen in the npm build step in CI:

(!) Plugin typescript: @rollup/plugin-typescript TS7015: Element implicitly has an 'any' type because index expression is not of type 'number'.
src/api/EpiData.ts: (186:98)

186             [f01] Failed to fetch API data from <a href="${url.href}">API Link</a>:<br/><i>${res['message']}</i>
                                                                                                     ~~~~~~~~~

(!) Plugin typescript: @rollup/plugin-typescript TS[13](https://github.com/cmu-delphi/www-epivis/actions/runs/13682197887/job/38257069595?pr=90#step:7:14)71: This import is never used as a value and must use 'import type' because 'importsNotUsedAsValues' is set to 'error'.
src/data/DataSet.ts: (3:1)

3 import EpiPoint from './EpiPoint';
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

(!) Plugin typescript: @rollup/plugin-typescript: Typescript 'sourceMap' compiler option must be set to generate source maps.
(!) Circular dependency
src/store.ts -> src/deriveLinkDefaults.ts -> src/api/EpiData.ts -> src/store.ts