feat(core,view): ③ 공통 설정 — fck and fy as inputs (spec §6.2)

- Material {fck, fyH, fyS}, defaults 27 / 400 / 500 as assumed values (I02).
  The rebar calculations take the material: the C7/C8 splice, the P2 development
  length, the P3 lap, and the B2 split use it instead of fixed numbers.
- ③ screen: 공통 설정 inputs with a note that the defaults are assumed (I02).
- Project file stores the material as an optional field; older files open with
  the defaults.
- Tests: fck 27 → 35 shortens the laps by √(27/35); the C7 piece follows fck;
  a non-default fy drops I02; the file round trip.

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>

src/style.css

@@ -686,4 +686,8 @@ body { background: #eef1f4; font-family: 'Noto Sans KR', system-ui, sans-serif;
 .rebar-table td:nth-child(5),
 .rebar-table th:nth-child(5) { text-align: left; }
 .rebar-table td:nth-child(5) { white-space: normal; min-width: 260px; }
+/* ③ 공통 설정 */
+.common { margin: 10px 0; display: grid; gap: 6px; }
+.mat { display: flex; justify-content: space-between; align-items: center; gap: 8px; font-size: 13px; }
+.mat input { width: 96px; padding: 4px 6px; border: 1px solid #b9c4d0; border-radius: 4px; }