fix(desktop): keep the framed titlebar identity clear of native caption controls

[wuzao368]

Problem

On Windows (compatibility / extended modes), when the main window is narrow, the centered title-bar identity row — product name, version pill, and presentation-mode pill — overflows into the rightmost native caption-controls strip reserved for the titleBarOverlay buttons. Because the overlay background is fully transparent (#00000000), the version/mode pills remain visible underneath the native minimize / restore / close glyphs, which renders as colliding or overlapping buttons.

The overlap happens whenever the identity's natural width (≈210–270px) exceeds windowWidth − 292px (below roughly 370–430px window widths). Restored narrow bounds from older builds (persisted main-window-state.json) or high-DPI configurations make this easy to hit.

Root cause

src/client/extended-styles.ts positions the identity with:

.dshDesktopFrameIdentity {
  position: absolute;
  left: 50%;
  transform: translateX(-50%);
  /* no width cap — content overflows into the native-controls strip */
}

The frame is viewport-centered while the frame padding is asymmetric (0 146px 0 8px on win32), so the centered box needs a cap that reserves the full native strip on both sides: 2 × (WINDOWS_CAPTION_CONTROLS_WIDTH + 8) = 292px on win32, 2 × (MACOS_TRAFFIC_LIGHT_SAFE_WIDTH + 8) = 176px on macOS.

Fix

• Cap .dshDesktopFrameIdentity to the visible title-bar band per platform (max-width: calc(100% - 292px) / calc(100% - 176px)), so it can never reach the native controls at any window width. The identity always stays within [146, W−146] on Windows — 8px clear of the overlay strip on the right, clear of the action buttons on the left.
• Let identity children shrink (min-width: 0) and truncate with an ellipsis instead of spilling.
• Hide the decorative product label below 480px so the interactive version/mode pills stay readable on narrow windows.

Supported window widths (≥ 640px) are unaffected — the cap only engages below ~566px.

Verification

• yarn workspace dsh-plugin-desktop typecheck — passes (all 5 tsconfigs).
• vitest run tests/client-environment.spec.ts — 20/20 pass, including new regression assertions for the identity cap, child shrink/ellipsis rules, and the narrow-width media query.
• Geometry simulation across 324/400/480/560/640/900/1280px widths: the identity span [146, W−146] never intersects the native overlay zone [W−138, W].

Notes

• The auxiliary native-ui frame header (dshNativeFrame) is an empty drag strip and the advanced-mode caption row is empty (aria-hidden), so neither has this surface.
• Screenshot of the original overlap: title bar shows the mode pill colliding with the native restore button at a ~324px-wide window.