Add slash command autocomplete popup

Typing / in the chat composer now shows a filtered popup listing all
available commands with their description. Arrow keys or Tab to select,
Enter/Tab to insert, Esc to close, click also works.

- New module: static/js/slashAutocomplete.js
  Reads the existing COMMANDS registry (and LEGACY_ALIASES) from
  slashCommands.js — no command logic added here, just discovery UI.
  Excludes easter-egg commands (flip, roll, 8ball, fortune, odyssey,
  ascii). Promotes short legacy aliases (/new, /clear, /web, /compact,
  /research, etc.) as first-class rows so users don't have to know the
  full /session new form.

- slashCommands.js: export COMMANDS and LEGACY_ALIASES so the new
  module can read the registry.

- chat.js: lazy-import slashAutocomplete on init, wire to #message
  textarea.

- style.css: popup + row styles using existing CSS variables.

static/style.css

@@ -34358,3 +34358,68 @@ body.theme-frosted .modal {
   background-color: color-mix(in srgb, var(--accent, var(--red)) 10%, transparent);
   transform: translateX(1px);
 }
+
+/* Slash command autocomplete popup, anchored to the message composer */
+.slash-autocomplete-popup {
+  position: fixed;
+  z-index: 9000;
+  background: var(--bg-elev-2, #1a1a1a);
+  border: 1px solid var(--border, rgba(255,255,255,0.08));
+  border-radius: 8px;
+  box-shadow: 0 8px 24px rgba(0,0,0,0.35);
+  font-size: 13px;
+  color: var(--fg, #e6e6e6);
+  overflow-y: auto;
+  padding: 4px 0;
+  display: none;
+}
+.slash-ac-cat {
+  font-size: 10px;
+  letter-spacing: 0.08em;
+  text-transform: uppercase;
+  color: var(--fg-muted, #888);
+  padding: 6px 10px 2px;
+  opacity: 0.7;
+}
+.slash-ac-row {
+  display: flex;
+  align-items: baseline;
+  gap: 8px;
+  padding: 5px 10px;
+  cursor: pointer;
+  line-height: 1.3;
+  white-space: nowrap;
+  overflow: hidden;
+}
+.slash-ac-row:hover { background: color-mix(in srgb, var(--fg) 6%, transparent); }
+.slash-ac-row-sel  { background: color-mix(in srgb, var(--accent, var(--red)) 14%, transparent); }
+.slash-ac-token {
+  font-family: 'Fira Code', ui-monospace, monospace;
+  color: var(--accent, var(--red));
+  font-weight: 600;
+  flex-shrink: 0;
+}
+.slash-ac-help {
+  color: var(--fg);
+  opacity: 0.85;
+  flex: 1;
+  min-width: 0;
+  overflow: hidden;
+  text-overflow: ellipsis;
+}
+.slash-ac-usage {
+  color: var(--fg-muted, #888);
+  font-family: 'Fira Code', ui-monospace, monospace;
+  font-size: 11px;
+  opacity: 0.55;
+  flex-shrink: 0;
+}
+.slash-ac-empty {
+  padding: 10px;
+  color: var(--fg-muted, #888);
+  font-style: italic;
+}
+.slash-ac-empty code {
+  font-family: 'Fira Code', ui-monospace, monospace;
+  color: var(--accent, var(--red));
+}