feat(browser): preserve page appearance while native views are occluded

[wsp1911]

Summary

• Show a cached, decoded page snapshot beneath floating surfaces when the native browser is hidden.
• Align native views and snapshots to the same physical-pixel rectangle, preventing rounded corners and subtle shifts during transitions.
• Bound screenshot size, invalidate stale frames, and retain the existing placeholder when capture is unavailable.

Type and Areas

Type: Feature / bug fix / UI/UX

Areas: Desktop/Tauri, Web UI, remote surface contracts, docs

Motivation / Impact

Opening a tooltip, context menu, or session overview over the embedded browser previously exposed a generic placeholder. Cached snapshots preserve the page appearance while allowing floating surfaces to remain visible.

Snapshots now use square corners and matching native-view geometry, avoiding visible movement at fractional display scaling.

Verification

• Passed: pnpm run check:web.
• Passed: pnpm --dir src/web-ui exec vitest run src/app/scenes/browser/browserPreviewCache.test.ts src/app/scenes/browser/nativeWebviewVisibility.test.ts src/app/scenes/browser/useEmbeddedBrowserWebview.test.tsx — 18 tests.
• Passed after removing diagnostic probes: pnpm --dir src/web-ui exec tsc --noEmit and git diff --check.
• Manual Windows desktop validation at 150% scaling: the user confirmed the fix after reproducing transitions through a FlowChat file-link tooltip, browser-tab context menu, and session overview.
• Rust tests and macOS/Linux runtime checks were not run in this verification pass. Real remote scenarios were not exercised.

Reviewer Notes

• Capture runs only while the native view and document are visible; hiding never waits for capture.
• Frames remain in memory, with the longest edge limited to 1600 pixels.
• Preview capture stays controller-local in Peer Device Mode. The web iframe path is unchanged.
• Unsupported or older hosts retain the existing placeholder.
• Temporary diagnostic probes were removed after user confirmation.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable.