* ✨ Feature(gui): configure server upload secret in settings
PicGo Core already supports authenticating `POST /upload` with a shared secret (`Authorization: Bearer <secret>`, `X-PicGo-Secret` or `?secret=`), resolved from `PICGO_SERVER_SECRET` or `settings.server.secret`, but the GUI offered no way to set it. Add a masked secret field with a show/hide toggle to the PicGo Server dialog, next to host and port.
The dialog saves the trimmed value as `settings.server.secret`; an empty value disables authentication. Saving already restarts the server, and Core reads the secret from config on `listen()`, so the main process needs no change and `PICGO_SERVER_SECRET` still takes precedence. The secret value is never added to the settings search index.
* 🐛 Fix(gui): keep server secret when normalizing settings
normalizeSettingsConfig only copied host, port and enable from `settings.server`, so after a reload the renderer state had no secret: the server dialog reopened with an empty secret field, and saving it again (e.g. after changing the port) would have silently cleared the secret and disabled authentication. Keep `secret` (falling back to an empty string) and cover it with a test.
* 📝 Docs: document renderer config normalizer whitelist in AGENTS.md
The server secret was dropped from renderer state because normalizeSettingsConfig only copies listed fields, and the server dialog re-saves the whole object. Record the rule for future changes: when adding a persisted config field, update the shared types, renderer state type, default value and normalizer, and add a round-trip normalizer test. Also give `settings.server.secret` an explicit empty default to follow that rule.
* 📝 Docs: require English PRs and squash merges in AGENTS.md
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