`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 e0ccdb4d9). 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
a1bb958b18