fix(browser-core): explain overlay-hidden webviews

[Harry19081]

Problem

Opening a menu or dialog over an active native browser can park the webview offscreen or place it behind the React layer. The visible BrowserCore pane then appears blank with no explanation because overlay blocking and native webview ownership are not projected into the browser chrome.

Solution

Render a non-interactive placeholder while a nonblank, visible BrowserCore pane has its native webview hidden by overlay state. The condition uses the overlay-only blocking atom plus the active overlay count for the macOS layering path, while excluding blank tabs, host-hidden panes, and the hidden owner host. Add localized title/body copy for every supported locale and regression coverage for the blocking, macOS, shared-runtime, and exclusion paths.

Potential risks

The notice is driven by global overlay state, so a future overlay that does not cover the browser could surface the message more broadly than intended. The exclusions and platform paths are unit-tested, and the change does not alter persisted data, public APIs, IPC, or wire formats. Native Tauri visual behavior remains unverified, so this PR is intentionally a draft.

Verification

• git diff --check — passed
• pnpm run lint — passed with zero warnings
• pnpm run typecheck — passed
• pnpm run check:circular — passed; no circular dependencies across 6,566 modules
• pnpm run test — passed; 1,229 files and 9,773 tests, including all 10 BrowserCore tests
• Commit hooks — lint-staged and scoped TypeScript checks passed
• Not run: native Tauri visual verification on macOS or other platforms

UI evidence

No screenshot was captured because running and inspecting the desktop UI would require explicit desktop-control authorization. The PR remains draft until the native overlay paths and relevant themes are checked in a running Tauri build.