feat: the Discord Add panel's artist field autocompletes against FabledCurator's artists (milestone 429)

Operator, after the first live run: "I need the extension to offer an
autofill search function so it's easier to match an entry with an existing
artist."

- The field searches as soon as the panel opens (the server name, usually)
  and on every keystroke; matches list under it, with ↑/↓, Enter/Tab to pick,
  Esc to close, and a last "+ New artist" row.
- Inline autofill: the rest of the top match is filled in and selected, so
  typing on replaces it and Tab/Enter accepts it. Backspacing never refills.
- A result whose name IS the text, spacing and case aside, is picked on its
  own; the hint says in green which existing artist the source will join.
- /api/artists/autocomplete also matches ignoring spacing and punctuation
  ("Tamada Heijun" finds "TamadaHeijun"), ranked just below an exact match.
  The web UI's artist search gets it too.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

extension/content/content-script.css

@@ -82,3 +82,18 @@
 }
 .fc-panel__btn--primary { border-color: rgb(244, 186, 122); background: rgb(244, 186, 122); color: rgb(20, 23, 26); }
 .fc-panel__btn:disabled { opacity: 0.5; cursor: default; }
+
+/* Artist autocomplete — the list sits under the field, inside the panel. */
+.fc-panel__combo { position: relative; }
+.fc-panel__results {
+  margin-top: 4px; border-radius: 6px;
+  background: rgb(12, 14, 16);
+}
+.fc-panel__results:empty { display: none; }
+.fc-panel__results:not(:empty) { border: 1px solid rgb(70, 74, 80); padding: 3px; }
+.fc-panel__result { display: flex; justify-content: space-between; align-items: center; width: 100%; box-sizing: border-box; }
+.fc-panel__result--active { background: rgb(52, 44, 32); outline: 1px solid rgb(244, 186, 122); }
+.fc-panel__result--new { color: rgb(244, 186, 122); }
+.fc-panel__result-tag { font-size: 11px; color: rgb(140, 220, 160); }
+.fc-panel__empty { padding: 6px 9px; font-size: 12px; color: rgb(170, 166, 156); }
+.fc-panel__hint--match { color: rgb(140, 220, 160); }