fix(ui): DPI-scale hand-drawn overlays + freeze sweep demo state

Hand-drawn absolute geometry (dl->AddText offsets, SetNextWindowSize,
explicit ImVec2 button widths, SameLine strides) is immune to the app's
DPI machinery (font-atlas rebuild + ScaleAllSizes), so several overlays
rendered native-size (tiny) on HiDPI / Windows 150% displays. Multiply the
raw logical-px geometry by Layout::dpiScale():

- seed_display: content-aware, DPI-scaled seed-grid column stride (a raw
  130px stride let words collide with the next column's index at 150%)
- lock screen: card/logo/input/unlock-button + all vertical offsets
- migrate-to-seed + seed-backup dialogs: button widths
- send-confirm: confirm/cancel button widths
- about dialog: value-column x, edition inset, link/close button widths
- chat conversation list: row height + padding (fixes the preview clip)

Also freeze demo state during the full UI sweep: guard App::update() on
capture_mode_ so a running daemon's refresh can't clobber the injected
demo state (it blanked Send/Receive/History with a CDB error mid-sweep).

Verified on Windows 4K@150% and locally at font_scale=1.5 (same
dpiScale()==1.5 code path). Adds a DPI-scaling convention note to CLAUDE.md.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -106,5 +106,6 @@ The version has a **single source of truth**: `project(... VERSION 1.2.0 ...)` p
 - **C++17.** Match the surrounding code's style per file.
 - **Icons:** use the Material Design icon font defines (`ICON_MD_*`); never raw Unicode glyphs.
 - **UI layout values** belong in `res/themes/ui.toml`, read via `schema::UI()` — do not hardcode pixel sizes/offsets in code.
+- **DPI / display scaling:** `schema::UI()` returns **logical (raw) px**; `Layout::dpiScale()` (= OS DPI × the in-app font-scale) is the single scale factor. The `Layout::k*()` helpers and `BeginOverlayDialog` already fold it in, and ImGui auto-layout scales via the DPI-rebuilt font atlas + `ScaleAllSizes` — so tabs/standard widgets scale for free. But **hand-drawn absolute geometry** (`dl->AddText` at manual `cy += 24.0f` offsets, `SetNextWindowSize`, explicit `ImVec2(width,0)` button sizes, `SameLine(x)` column strides) is immune to all of that and must be multiplied by `ui::Layout::dpiScale()` yourself, or it renders native-size (tiny) on a HiDPI display. Font metrics (`font->LegacySize`, `CalcTextSize`) are already scaled — don't double-scale those. Verify at scale with a full sweep run at `font_scale: 1.5` (same `dpiScale()==1.5` code path as OS 150%).
 - **i18n:** user-facing strings are translated via `src/util/i18n`; the English source of truth is the `strings_[...]` map in `src/util/i18n.cpp`, and the per-language translations live as the **source of truth** in `res/lang/` (`de`, `es`, `fr`, `ja`, `ko`, `pt`, `ru`, `zh`). **Edit those JSONs directly and additively** — write with `json.dump(..., indent=4, sort_keys=True, ensure_ascii=False)`; never bulk-regenerate/overwrite a whole file (that path silently dropped ~285 keys/language before). `scripts/add_missing_translations.py` back-fills only the keys missing from a JSON (non-destructive), and `scripts/build_cjk_subset.py` rebuilds the CJK subset font (`res/fonts/NotoSansCJK-Subset.ttf`) after new CJK glyphs are added.
 - **Commits:** the history uses Conventional Commits (`feat(scope): …`, `fix(scope): …`). PRs target `master`.