Settings (4/4): restructure the settings drawer; translated server errors (#160) #628

Split from #355 (the whole issue does not fit one run). Common rules: tests with teeth, Playwright for UI behaviour (CLAUDE.md §3.3). Platform owner preferences (freeshard#168): use them as defaults if the platform offers them; otherwise default from the browser (navigator.language, Intl time zone) and note the fallback in CONCEPTS.md — do not block on the platform. Builds on #627.

Acceptance criteria

• The settings drawer (7 sections, inline styles) becomes a structured settings page/dialog with sections per topic, design tokens instead of inline styles, keyboard and screen-reader friendly (dialogFocus).
• Server errors reach the UI as codes with parameters and are shown translated (HIGH: every failure below account setup is reported in English protocol jargon, with the upstream response body pasted in #160); no raw English server message in a toast.
• Playwright for navigation through the settings and for a translated server error in both languages.