feat(share): symbol page share button + distinguish note delete vs failure

- New ShareButton in the page header tries navigator.share() first
  (native sheet on mobile), then falls back to navigator.clipboard, then
  to a hidden textarea + execCommand("copy") for non-secure-context LAN.
  Shows a brief "링크 복사됨" / "복사 안됨" toast; native share success
  shows none (the OS sheet is the feedback).
- Mounted between ⇄ 비교 and the Star button.

Fixes reviewer concern on 442cc38: writeNote() used to return null for
both "user emptied the textarea (intentional delete)" and "localStorage
quota failure," so clearing a note showed "저장 안됨." It now returns a
discriminated { status: "saved" | "deleted" | "failed" } and the panel
shows "저장됨" / "삭제됨" / "저장 안됨" accordingly. Failure keeps the
user's text so they can retry or shorten it.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

web/components/InvestmentNote.tsx

@@ -7,6 +7,12 @@
 import { useEffect, useMemo, useState } from "react";
 import { NOTE_MAX_LEN, readNote, writeNote, type Note } from "../lib/notes";
 
+const FEEDBACK_TEXT: Record<"saved" | "deleted" | "failed", string> = {
+  saved: "저장됨",
+  deleted: "삭제됨",
+  failed: "저장 안됨",
+};
+
 type Props = {
   code: string;
 };
@@ -60,10 +66,16 @@ export function InvestmentNote({ code }: Props) {
   const remaining = useMemo(() => NOTE_MAX_LEN - text.length, [text]);
 
   const save = () => {
-    const next = writeNote(code, text);
-    setSaved(next);
-    setText(next?.text ?? "");
-    setFeedback(next ? "저장됨" : "저장 안됨");
+    const res = writeNote(code, text);
+    if (res.status === "saved") {
+      setSaved(res.note);
+      setText(res.note.text);
+    } else if (res.status === "deleted") {
+      setSaved(null);
+      setText("");
+    }
+    // failed: 입력값 유지 — 사용자가 재시도하거나 일부를 줄일 수 있도록.
+    setFeedback(FEEDBACK_TEXT[res.status]);
   };
 
   return (