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/en-US.json

@@ -153,6 +153,11 @@
       "currentPanelVersion": "Current panel version",
       "latestPanelVersion": "Latest panel version",
       "panelUpToDate": "Panel is up to date",
+      "devChannel": "Dev channel",
+      "devChannelWarning": "Dev builds track every commit on main and aren't stable releases — there is no automatic downgrade.",
+      "currentCommit": "Current commit",
+      "latestCommit": "Latest commit",
+      "updateChannelChanged": "Update channel changed",
       "upToDate": "Up to date",
       "xrayStatusUnknown": "Unknown",
       "xrayStatusRunning": "Running",