fix(ui): remove opaque corner fillets on transparent window

[thanhleviet]

Summary

• Fix opaque rectangular fillets visible at four window corners in dark theme
• Window now renders clean rounded corners by compositing directly against the desktop

Problem

The window had transparent: false in Tauri config, so macOS drew an opaque rectangular frame behind the CSS border-radius: 14px on #overlay-view and #settings-view. This made the four corner areas visibly opaque — appearing as small rectangular "fillets" that broke the rounded corner appearance.

Fix

• transparent: true — makes the native window background clear so CSS rounded corners show through to the desktop
• shadow: false — disables the macOS system window shadow, which is rectangular and would peek out behind the rounded CSS shape
• clip-path: inset(0 round 14px) — hard-clips each view's rendering (including backdrop-filter blur which can leak outside border-radius on WebKit) to the rounded rectangle
• Removed outer box-shadow from both views — on a transparent window, the dark drop shadows would float visibly in the transparent corner area

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Beautiful app — the glassmorphism design, clean TTS provider abstractions, and the vanilla JS + Tauri architecture make this a joy to work with.