Add full-featured AIChat component

AIChat (elements/ai/AIChat) is a complete LLM chat UI built on new hooks:
- useAIChat: streaming conversation state (send, stop, regenerate,
  edit, feedback); `send` may return a string, async iterable,
  ReadableStream or fetch Response
- useAIChatHistory: saved conversations with optional localStorage
  persistence and generated titles, shown in AIChatSidebar
- useAIChatSpeech: read aloud via browser speech synthesis or a
  custom `speak` returning audio
- useAIChatDictation: voice input via browser recognition or a
  custom `transcribe`

Also adds a model picker, message editing, thumbs up/down, markdown
export, scoped markdown styles in base.css, the twuiReadTextStream,
twuiStripMarkdown and twuiGroupByRecency utils (with tests), and a
playground page. AIPromptBlock is kept and marked deprecated.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/utils/group-by-recency.ts

@@ -0,0 +1,46 @@
+export type TWUIRecencyGroup<T> = {
+    label: string;
+    items: T[];
+};
+
+const DAY = 24 * 60 * 60 * 1000;
+
+/**
+ * # Group by recency
+ * Buckets items into "Today", "Yesterday", "Previous 7 days",
+ * "Previous 30 days" and then by month, newest first - the grouping chat
+ * apps use for conversation lists. Empty groups are left out.
+ */
+export default function twuiGroupByRecency<T>(
+    items: T[],
+    getTime: (item: T) => number,
+    now: number = Date.now(),
+): TWUIRecencyGroup<T>[] {
+    const startOfToday = new Date(now);
+    startOfToday.setHours(0, 0, 0, 0);
+    const today = startOfToday.getTime();
+
+    const groups = new Map<string, T[]>();
+
+    const label = (time: number) => {
+        if (time >= today) return "Today";
+        if (time >= today - DAY) return "Yesterday";
+        if (time >= today - 7 * DAY) return "Previous 7 days";
+        if (time >= today - 30 * DAY) return "Previous 30 days";
+        return new Date(time).toLocaleDateString(undefined, {
+            month: "long",
+            year: "numeric",
+        });
+    };
+
+    [...items]
+        .sort((a, b) => getTime(b) - getTime(a))
+        .forEach((item) => {
+            const key = label(getTime(item));
+            const list = groups.get(key);
+            if (list) list.push(item);
+            else groups.set(key, [item]);
+        });
+
+    return Array.from(groups, ([label, items]) => ({ label, items }));
+}