#17920: macOS: fix WebChat panel corner clipping

Fixes #12294 ## Summary - fix bottom-corner rendering artifacts in the macOS WebChat panel - use `.withinWindow` blending for panel mode to avoid material bleed at rounded edges - enforce continuous corner clipping on the effect, hosting, and container views ## Verification - built and ran mac app via documented flow (`scripts/restart-mac.sh --no-sign`) - manually confirmed the bottom-corner UI glitch is resolved in WebChat panel ## Before <img width="494" height="36" alt="Screenshot 2026-02-16 at 10 31 38" src="https://github.com/user-attachments/assets/51d8251b-2341-46fe-9cb6-11218ad3d4b3" /> ## After <img width="483" height="45" alt="Screenshot 2026-02-16 at 10 26 05" src="https://github.com/user-attachments/assets/aad42cc1-53d6-418a-b507-24d6db7cef40" /> <!-- greptile_comment --> <h3>Greptile Summary</h3> Fixes bottom-corner rendering artifacts in the macOS WebChat panel by switching `NSVisualEffectView` blending mode from `.behindWindow` to `.withinWindow` in panel mode, adding continuous corner clipping to the hosting view, and setting a clear background color on the effect view layer. - Conditionally sets `blendingMode` based on presentation (`.withinWindow` for panel, `.behindWindow` for window) to prevent material bleed at rounded edges - Applies matching `cornerCurve`, `cornerRadius`, and `masksToBounds` to the `NSHostingController` view so the SwiftUI content also clips to the rounded corners - Adds explicit `NSColor.clear.cgColor` background to the effect view layer - Removes dead code that was setting layer properties on `controller.view` before it was overwritten by `controller.view = rootView` <h3>Confidence Score: 5/5</h3> - This PR is safe to merge — it makes a focused, low-risk UI fix with no behavioral or API changes. - The changes are limited to a single file, touch only view layer properties for corner clipping, and follow existing patterns in the codebase. The blending mode change is presentation-specific and preserves existing window-mode behavior. The dead code removal is correct and was identified in prior review. No logic, security, or regression concerns. - No files require special attention. _{Last reviewed commit: cc9f099} <!-- greptile_other_comments_section --> <!-- /greptile_comment -->