feat(update): add rolling dev update channel for per-commit builds

Adds an opt-in Dev channel so panels running CI per-commit builds can self-update to the latest commit, mirroring the stable online-update flow.

CI publishes/overwrites a single fixed-tag pre-release (dev-latest), force-moved to the newest main commit and marked --latest=false so releases/latest stays the stable tag. Builds stamp the short commit via -ldflags; the panel compares the running commit to the dev release commit to detect an update, and update.sh honors XUI_UPDATE_TAG to install from that tag. Linux/systemd only.

internal/web/translation/ja-JP.json

@@ -153,6 +153,11 @@
       "currentPanelVersion": "現在のパネルバージョン",
       "latestPanelVersion": "最新のパネルバージョン",
       "panelUpToDate": "パネルは最新です",
+      "devChannel": "開発チャンネル",
+      "devChannelWarning": "開発ビルドは main の各コミットを追跡し、安定版ではありません。自動ダウングレードはありません。",
+      "currentCommit": "現在のコミット",
+      "latestCommit": "最新のコミット",
+      "updateChannelChanged": "更新チャンネルを変更しました",
       "upToDate": "最新",
       "xrayStatusUnknown": "不明",
       "xrayStatusRunning": "実行中",