fix(deck): don't wipe deck on unparseable bulk-replace input

lib/deck/io/__tests__/intake.test.ts

@@ -434,6 +434,26 @@ describe("intakeDecklist — replace mode", () => {
     expect(result.updated).toBe(0);
   });
 
+  it("aborts without wiping the deck when the paste fails to parse (issue #87)", async () => {
+    // Guard short-circuits before the deck is read or mutated, so no
+    // deckCard.findMany / applyChanges mocks are queued here on purpose.
+    const result = await intakeDecklist({
+      deckId: "deck-1",
+      userId: "user-1",
+      // Detected as JSON (leading "{") but not valid JSON → zero parsed cards.
+      text: "{ not valid json",
+      mode: "replace",
+    });
+
+    expect(mockApplyChanges).not.toHaveBeenCalled();
+    expect(mockDeckCardFindMany).not.toHaveBeenCalled();
+    expect(result.applied).toBe(0);
+    expect(result.removed).toBe(0);
+    expect(result.warnings).toContain(
+      "Decklist looks like JSON but could not be parsed",
+    );
+  });
+
   it("re-throws non-InvariantViolation errors from applyChanges", async () => {
     mockCardFindMany.mockResolvedValueOnce([
       { id: 1, name: "Lightning Bolt" },

lib/deck/io/intake.ts

@@ -155,6 +155,15 @@ export async function intakeDecklist(input: IntakeInput): Promise<IntakeResult>
   const warnings = [...resolved.warnings];
   const unmatchedNames = resolved.unmatched.map((c) => c.name);
 
+  // Replace mode diffs the parsed list against the entire deck, so a parse
+  // that yielded nothing would delete every card. A parse failure produces
+  // zero parsed cards *with* warnings — distinct from a legitimately empty
+  // paste (zero cards, no warnings), which may intentionally clear the deck.
+  // Abort with a warning-only result rather than silently wiping the deck.
+  if (mode === "replace" && parsed.cards.length === 0 && warnings.length > 0) {
+    return { applied: 0, added: 0, removed: 0, updated: 0, unmatchedNames, warnings };
+  }
+
   const changes =
     mode === "append" ? asAdds(resolved) : await buildReplaceChanges(deckId, resolved);