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/lib/notes.ts

@@ -24,20 +24,25 @@ export function readNote(code: string): Note | null {
   }
 }
 
-export function writeNote(code: string, text: string): Note | null {
-  if (typeof window === "undefined") return null;
+export type WriteResult =
+  | { status: "saved"; note: Note }
+  | { status: "deleted" }
+  | { status: "failed" };
+
+export function writeNote(code: string, text: string): WriteResult {
+  if (typeof window === "undefined") return { status: "failed" };
   const trimmed = text.slice(0, MAX_LEN);
-  // 공백만이면 키 자체 제거 → 빈 잔여물 방지.
+  // 공백만이면 키 자체 제거 → 빈 잔여물 방지. 의도된 삭제이므로 failed 와 구분.
   if (!trimmed.trim()) {
     clearNote(code);
-    return null;
+    return { status: "deleted" };
   }
   const next: Note = { text: trimmed, updatedAt: new Date().toISOString() };
   try {
     window.localStorage.setItem(`${PREFIX}${code}`, JSON.stringify(next));
-    return next;
+    return { status: "saved", note: next };
   } catch {
-    return null; // quota 등 — UI 가 저장 실패 처리
+    return { status: "failed" }; // quota 등 — UI 가 저장 실패 처리
   }
 }