style: Phase 1+2 — FA→Lucide, variant-* → preset-*, dark mode forms

Phase 1 (global quick wins):
- app.css: add global dark mode utility for .input/.select/.textarea
- events menu nav + layout: replace all FontAwesome icons with Lucide
- events settings: replace FA icons, standardize variant-* → preset-*

Phase 2 (module-by-module migration):
- root +layout.svelte: fix hardcoded banner colors → preset-filled-error/warning
- journals entry list: replace slate-* with gray-*, HSL eye colors → CSS tokens
- pres mgmt presenter view: variant-soft-warning → preset-tonal-warning, FA edit → Lucide
- badges (4 files): variant-* → preset-*, FA spinner → Lucide Loader2, dynamic alert fix
- events session list + event hub: variant-soft/filled → preset-tonal/filled
- leads module (12 files): complete variant-* → preset-* migration across all
  exhibitor sign-in, QR scanner, manual search, tracking list, manage tab,
  custom questions, license list, exhibit page, lead detail page + form

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/routes/events/ae_comp__event_session_obj_li.svelte

@@ -187,7 +187,7 @@
                                     {#if (show__session_presentations || show__session_files) && $ae_loc.manager_access}
                                         <button
                                             type="button"
-                                            class="btn btn-icon btn-sm variant-soft-surface"
+                                            class="btn btn-icon btn-sm preset-tonal-surface"
                                             onclick={() =>
                                                 toggle_details(
                                                     session_obj.event_session_id