feat: improve completions loading and UI handling

Refactor the completions system to support both CLI mode (via
`/__COMPLETIONS.json`) and a client-side fallback scan for dev mode.

- Implement client-side scanning of the file index for dice and headings
- Add `ensureCompletions` to allow components to await data readiness
- Update `JournalInput` to handle "no results" states in the dropdown
- Improve keyboard navigation (Tab to accept, Enter to select)
- Update dice regex to support `:md-dice[...]` syntax

src/cli/completions/sources/dice.ts

@@ -14,7 +14,7 @@ export const diceSource: CompletionSource = {
 
   scan(index) {
     const items: DiceCompletion[] = [];
-    const tagRegex = /<md-dice[^>]*>\s*([\s\S]*?)\s*<\/md-dice>/gi;
+    const tagRegex = /:md-dice\[([^[]+)\]/gi;
 
     for (const [path, content] of Object.entries(index)) {
       if (!path.endsWith(".md")) continue;