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/js/chat.js

@@ -156,6 +156,13 @@ import createResearchSynapse from './researchSynapse.js';
     initSlashCommands({ apiBase, isStreaming: () => isStreaming });
     // Initialize email inbox
     emailInbox.init(documentModule);
+    // Wire the slash-command autocomplete popup on the chat composer. The
+    // dispatcher already handles the typed command — this just surfaces the
+    // registry as a discoverable menu when the user starts a message with /.
+    import('./slashAutocomplete.js').then(mod => {
+      const ta = document.getElementById('message');
+      if (ta && mod.initSlashAutocomplete) mod.initSlashAutocomplete(ta);
+    }).catch(() => {});
   }
 
   // addMessage, createMsgFooter, displayMetrics, hideWelcomeScreen, showWelcomeScreen