Separate current/former members with calculable period dates

- Add MemberGroup/BandGroup types and groupBandMembers/groupArtistMembers helpers
- Calculate membership duration in months from YYYY-MM since/until values
- Band view splits members into 在籍中 / 元メンバー sections with duration label
- Artist view splits bands into 在籍中 / 元在籍 sections with duration label
- Change since/until inputs to type="month" for structured data entry

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/app/routes/band-edit.tsx b/app/routes/band-edit.tsx
index ffba70f..b64b707 100644
--- a/app/routes/band-edit.tsx
+++ b/app/routes/band-edit.tsx
@@ -271,15 +271,15 @@ export default function BandEdit() {
                         </div>
                         <div className="period-row">
                           <input
+                            type="month"
                             value={entry.since}
                             onChange={(e) => updateEntry(entry.key, "since", e.target.value)}
-                            placeholder="加入 (例: 2020-04)"
                           />
                           <span className="period-sep">〜</span>
                           <input
+                            type="month"
                             value={entry.until}
                             onChange={(e) => updateEntry(entry.key, "until", e.target.value)}
-                            placeholder="脱退 (空欄=在籍中)"
                           />
                           <input
                             className="period-note"