feat(settings): link the Steam account through Steam

A Steam card in the settings links, verifies or unlinks the Steam
account through "Sign in through Steam". After the way back the page
shows the outcome and refreshes the session, so Core gets the new
Steam ID at once. Registration and settings no longer ask for a typed
Steam ID.

public/i18n/en.json

@@ -23,14 +23,12 @@
     "PASSWORD": "Password",
     "PASSWORD_CONFIRMATION": "Confirm password",
     "INVITE_CODE": "Invite code",
-    "STEAM_ID": "Steam ID",
     "SUBMIT": "Create account",
     "SUCCESS": "Account created. Redirecting to login …",
     "HAVE_ACCOUNT": "Already have an account?",
     "ERROR_GENERIC": "Registration failed. Please try again.",
     "ERROR_EMAIL_IN_USE": "This email address is already registered.",
     "ERROR_INVALID_INVITE_CODE": "The invite code is invalid or has already been used.",
-    "ERROR_STEAM_ID_IN_USE": "This Steam ID is already linked to another account.",
     "DISPLAY_NAME": "Display name (optional)",
     "ERROR_DISPLAY_NAME_IN_USE": "This display name is already taken.",
     "ERROR_TOO_MANY_ATTEMPTS": "Too many failed attempts. Please try again in {{minutes}} minutes."
@@ -51,8 +49,6 @@
       "EN": "English",
       "DE": "German"
     },
-    "STEAM_ID": "Steam ID",
-    "SAVE_STEAM_ID": "Save Steam ID",
     "SAVED": "Settings saved.",
     "ERROR_SAVE": "Could not save settings.",
     "PROFILE": "Profile",
@@ -89,7 +85,23 @@
     "BACKGROUND_REMOVE": "Remove background",
     "BANNER": "Dashboard banner",
     "BANNER_HINT": "Shows your most played game of the last two weeks. Open a game and choose \"Use as banner\" to pick one.",
-    "BANNER_AUTO": "Back to automatic"
+    "BANNER_AUTO": "Back to automatic",
+    "STEAM_LINK": {
+      "LINK": "Link Steam account",
+      "VERIFY": "Verify with Steam",
+      "CHANGE": "Link another Steam account",
+      "UNLINK": "Unlink",
+      "VERIFIED": "Verified",
+      "NOT_VERIFIED": "Not verified",
+      "NONE": "No Steam account linked yet. You sign in on Steam, and Steam tells us your Steam ID; we never see your Steam password.",
+      "VERIFY_HINT": "This Steam ID was typed in and is not proved yet. Verify it with Steam, or another account that proves it takes it over.",
+      "LINKED": "Steam account linked.",
+      "IN_USE": "This Steam account is already linked to another account.",
+      "CANCELLED": "Steam link cancelled.",
+      "FAILED": "Steam could not confirm the link. Please try again.",
+      "ERROR": "Could not start the Steam link.",
+      "UNLINKED": "Steam account unlinked."
+    }
   },
   "VALIDATION": {
     "EMAIL_REQUIRED": "Email is required.",
@@ -98,8 +110,6 @@
     "PASSWORD_CONFIRMATION_REQUIRED": "Please confirm your password.",
     "PASSWORD_MISMATCH": "Passwords do not match.",
     "INVITE_CODE_REQUIRED": "Invite code is required.",
-    "STEAM_ID_REQUIRED": "Steam ID is required.",
-    "STEAM_ID_INVALID": "Steam ID must be a positive number.",
     "LANGUAGE_INVALID": "Unsupported language.",
     "DISPLAY_NAME_INVALID": "3–24 characters: letters, digits, spaces, _ . - (no space at the start or end)."
   },
@@ -113,7 +123,7 @@
     "LOADING": "Loading dashboard ...",
     "HELLO": "Hello",
     "STEAM_GAMES": "Steam Games",
-    "NO_STEAM_ID": "No Steam ID saved yet – add it in the settings",
+    "NO_STEAM_ID": "No Steam account linked yet – link it in the settings",
     "MOST_PLAYED": "Most played",
     "SHOW_ALL": "Show all",
     "TO_LIBRARY": "Go to library",