fix(pad): stop hardcoding lang='en', let the client auto-detect locale (#7586) (#7588)

`Pad.normalizePadSettings()` was defaulting `lang` to the literal string
'en' when `rawPadSettings.lang` was not a string. That value flowed into
`clientVars.padOptions.lang` and then into `getParams()` in pad.ts,
which calls `html10n.localize([serverValue, 'en'])` as a callback for
the `lang` setting. The result: every pad forced English on load,
overriding the browser's Accept-Language and the existing auto-detect
chain in l10n.ts (cookie -> navigator.language -> 'en').

The regression was introduced in #7545 ("Add creator-owned pad settings
defaults", commit e0ccdb4). 2.6.1 did not have this default, so
auto-detect worked there. 2.7.0 broke it.

Fix: default `lang` to null. The client's existing flow already handles
null correctly — getParams() at pad.ts:172 has
`if (serverValue == null) continue;`, so the forced-localize callback
simply does not fire, and l10n.ts's browser-language auto-detect runs.
Pad-settings dropdown consumer at pad.ts:489 already uses
`padOptions.lang || 'en'` so null renders fine there too.

`PadSettings.lang` is now typed `string | null` to match.

Added three backend regression tests under `normalizePadSettings lang`:
  * defaults to null when lang is absent (so client auto-detects)
  * preserves an explicit string lang (creator override still works)
  * drops non-string lang values to null rather than coercing to 'en'

Manual verification: with Firefox set to German, loading a fresh pad
now renders the UI in German. Index and timeslider continued to work
as before. Setting `?lang=de` or a language cookie continues to
override browser detection, as intended.

Fixes #7586

Author: JohnMcLear

src/node/db/Pad.ts

@@ -39,7 +39,7 @@ type PadSettings = {
   showChat: boolean;
   alwaysShowChat: boolean;
   chatAndUsers: boolean;
-  lang: string;
+  lang: string | null;
   view: PadViewSettings;
 };
 
@@ -91,7 +91,11 @@ class Pad {
         !!rawPadSettings.showChat,
       alwaysShowChat: !!rawPadSettings.alwaysShowChat,
       chatAndUsers: !!rawPadSettings.chatAndUsers,
-      lang: typeof rawPadSettings.lang === 'string' ? rawPadSettings.lang : 'en',
+      // Default to null (not 'en') so the client's l10n auto-detect chain
+      // (cookie -> navigator.language -> 'en' fallback) runs. Hardcoding 'en'
+      // forces English on every pad regardless of the browser's Accept-Language
+      // and broke #7586 (German system saw English pad UI in v2.7.0).
+      lang: typeof rawPadSettings.lang === 'string' ? rawPadSettings.lang : null,
       view: {
         showAuthorColors: rawView.showAuthorColors == null ? true : !!rawView.showAuthorColors,
         showLineNumbers: rawView.showLineNumbers == null ?

src/tests/backend/specs/Pad.ts

@@ -159,4 +159,23 @@ describe(__filename, function () {
       assert.equal(pad!.text(), `${want}\n`);
     });
   });
+
+  describe('normalizePadSettings lang (issue #7586)', function () {
+    it('defaults lang to null when not provided, so client auto-detects locale', function () {
+      const ps = Pad.Pad.normalizePadSettings({});
+      assert.equal(ps.lang, null);
+    });
+
+    it('preserves an explicit string lang (creator override)', function () {
+      const ps = Pad.Pad.normalizePadSettings({lang: 'de'});
+      assert.equal(ps.lang, 'de');
+    });
+
+    it('drops non-string lang values to null rather than coercing to "en"', function () {
+      for (const bogus of [42, true, {}, [], null, undefined]) {
+        const ps = Pad.Pad.normalizePadSettings({lang: bogus});
+        assert.equal(ps.lang, null, `bogus input ${JSON.stringify(bogus)}`);
+      }
+    });
+  });
 });