ui: the last of Semantic UI, in name and in dead CSS

The library has been uninstalled for some time, but the codebase still read as
though it were there: identifiers named after it, rules written against its
class names, and about 180 comments explaining a rule in terms of a thing that
is no longer installed.

Renamed, keeping the vocabulary the app actually uses: semanticUIColorMap ->
navColorHexMap, semanticColorNames -> paletteColorNames, semanticNames ->
iconNameAliases, semanticSizes -> sizeAliases, and SemanticUIColors ->
NavColors in wrolpi/schema.py, which is what validates nav_color on the way in.
ColorToSemanticHexColor had no callers at all and is gone.

The baseline ratchet has done its job -- the list reached zero, so the JSON is
deleted and the guard keeps only the checks with teeth: no package, no import,
no `className="ui "`, no toast library.  Its Theme.tsx check went with the
wrappers it was watching for.  It also asserts its own walk found files, so a
clean result cannot mean it scanned nothing.

Deleted from App.css: every rule requiring `.ui`, `.inverted`, `.segment` or
`.modal`, none of which any component has set since the stylesheet stopped
loading, plus the form skeletons, ui-alerts and progress-text rules that
nothing renders.  Two of them were doing harm rather than nothing: the drag
tints had a dark-theme variant under `.inverted` that stopped applying, leaving
15% of a fixed blue over a dark row, and now mix a token instead.

Two selectors naming markup that no longer exists were live bugs.  The file
browser's filter had `.ui.input.file-browser-filter`, so it rendered at the
browser default width on a bar laid out to give it 34em -- now qualified by its
container, because `.wrolpi-action-input` sets `width: 100%` at the same
specificity and is loaded later.  And SearchBox takes named props rather than a
rest spread, so the `id` DestinationForm hands it was dropped in silence and
the label it renders pointed at nothing; the download forms' Destination field
has had no associated label since the migration.

"Semantic roles" survives in four places, where it means what it says.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

app/src/components/Search.js

@@ -247,7 +247,7 @@ export function useSearchSuggestions(defaultSearchStr, defaultTagNames, anyTag)
     const noResults = [{title: 'No results'}];
 
     const normalizeSuggestionsResults = (newSuggestions) => {
-        // Convert the suggestions from the Backend to what the Semantic <Search> expects.
+        // Convert the suggestions from the Backend to what SearchBox expects.
         const lowerSearchStr = searchStr ? searchStr.toLowerCase() : '';
 
         let results = {};
@@ -516,8 +516,8 @@ export function SearchIconButton() {
     /*
      * A single button which opens the search modal via keyboard shortcuts context.
      *
-     * `wrolpi-navbar-link`, not Semantic's leftover `item`, which carried no rules at all
-     * once Semantic was removed: no pointer cursor, no hover highlight, and a hit area of
+     * `wrolpi-navbar-link`, not the leftover `item` class, which carries no rules at all
+     * now that the stylesheet defining it is gone: no pointer cursor, no hover highlight, and a hit area of
      * whatever the 18px glyph occupied rather than the height of the bar.  It is the same
      * kind of thing as Help and Admin and now has the same affordance.
      */