refactor(ui): give the icons one vocabulary and sweep the call sites

`plus` meant "add to the queue", "add to a playlist", "make a new
playlist" and "you do not own this" -- the first two adjacent in the
same context menu, so two neighbouring items were the same glyph doing
different things. `list` meant the queue (the button that opens it), the
Playlists destination, and adding to the queue in `queue-panel` alone.
Two icons carrying seven meanings is not a vocabulary, and nothing
catches it: a wrong-but-real icon renders perfectly.

`utils/icon-language.ts` is the table, beside `library-status.ts` as the
issue suggested. The rule it is built on is that an icon names the
**noun** it acts on, not the verb: "add to queue" and "add to playlist"
are one verb on two nouns, so the noun is what differs -- which is why
adding to a playlist wears the Playlists destination's own icon, and why
the queue took `bars-staggered` and stopped wearing Playlists'. `plus`
keeps the one meaning it is unambiguous about, making something that is
not there yet, which covers New Playlist and the drop zones.

`bars-staggered` is the only new glyph, vendored through names.txt and
fetch-icons.mjs after confirming it is in Font Awesome **Free** 7.3.1.

Two things this found rather than changed:

- The request toggle's outline/solid pair was already in the app and
  already right -- `explore-album-details`'s "Request this" button has
  used `regular/bookmark` -> `solid/bookmark` since it was written --
  while the badge forty pixels away showed a **plus** for the same
  state. That is `utils/library-status.ts`'s fault one layer down: it
  made the two surfaces agree on what wanting *means* and left them
  disagreeing on what it looks like.
- `explore-artist-details`'s Follow button was `bookmark-check`, which
  is Font Awesome **Pro** and has never been bundled, so it has drawn
  the missing-icon fallback -- a circled question mark -- for every
  followed artist since it was written. `requested-badge.spec.ts` was
  written for exactly this bug on the album button and says so in its
  docstring; this is the same bug one component over, still live,
  because `offline-icons.spec.ts` sweeps `__yjIconMisses` and no spec
  had ever followed an artist.

So the test does what reaching the state cannot. `icon-language.test.ts`
reads every `src/**/*.ts` as raw text and fails on a governed name
written outside the table, and separately asserts every `ICON_*` is a
*bundled* name -- which is what makes a Pro name a failing test rather
than a runtime report from a state something has to reach first. Its
first assertion is that it read any source at all, because a sweep over
an empty glob passes.

`chrome.test.ts` asserted `['check', 'bookmark', 'plus']` and so pinned
the badge's glyphs against the vocabulary they were meant to follow; it
names them from the table now, and keeps the assertion that the three
differ, which is the property the states actually need.

Downloads keeps the solid bookmark on purpose. That is one word twice,
not two words: the badge says the entity is on your list and the nav
item is that list.

Closes #34

frontend/src/components/library-status-indicator/library-status-indicator.ts

@@ -4,6 +4,11 @@ import '@awesome.me/webawesome/dist/components/icon/icon.js';
 import { toggleRequest } from '@utils/library-status';
 import { notificationStore } from '@store/notification-store';
 import { describeError } from '@utils/describe-error';
+import {
+    ICON_CAN_REQUEST,
+    ICON_IN_LIBRARY,
+    ICON_REQUESTED,
+} from '@utils/icon-language';
 
 /**
  * Library status for an entity (artist, album, or track).
@@ -248,18 +253,25 @@ export class LibraryStatusIndicator extends LitElement {
      * hourglass says "wait, this is under way", which overstates what a
      * request is: nothing may be downloading, nothing may ever be found,
      * and the user can leave one sitting on the list indefinitely. A
-     * bookmark says the honest thing — it is on your list — and reads as
-     * the opposite of the plus that put it there, which is what a
-     * toggle's two states have to do.
+     * bookmark says the honest thing — it is on your list.
+     *
+     * The *other* state is the outline of that same bookmark, not a
+     * plus. Two states of one toggle have to read as each other's
+     * opposite, and a plus and a bookmark do not — this badge showed a
+     * plus on the same page as a "Request this" button already using
+     * the outline/solid pair, forty pixels away. That is the fault
+     * `utils/library-status.ts` was written for, one layer down: it
+     * made the two agree on what wanting *means* and left them
+     * disagreeing on what it looks like.
      */
     private iconName(): string {
         switch (this.status) {
             case 'in-library':
-                return 'check';
+                return ICON_IN_LIBRARY;
             case 'queued':
-                return 'bookmark';
+                return ICON_REQUESTED;
             default:
-                return 'plus';
+                return ICON_CAN_REQUEST;
         }
     }