fix(ui): homepage previews, theme glass, folder open, ClickGUI clip

[gaoyu06]

Fixes Edge homepage background previews, theme switching (window stayed black), both “打开文件夹” actions, ClickGUI expanded-settings overflow, and locks Fast Render off by default.

What changed

• Fast Render default: already false; FastRenderDefaultTest locks constructor + source default so enabling Performance does not turn it on.
• Homepage preview cards / theme glass: EdgeCanvas.strokeRoundRect faked a stroke by filling a larger rounded rect, then cutting the interior with a hardcoded dark color. That painted over panorama previews and Theme.glass(). Stroke is now a ring (Rects.roundedOutline).
• 打开文件夹 (background + music): FolderOpen uses explorer.exe / open / xdg-open, then on Windows raises Explorer via JNA. Missing background.png is treated as a file so the parent client dir opens.
• ClickGUI clip: Scissor.toFramebuffer uses ScaledResolution / UiScale, flips Y, clamps. Every pushClip is intersected with the SharedClickGui panel so expanded settings stay inside the window.

Verification

• ./gradlew test — 49 tests, 0 failures
• ./gradlew build — BUILD SUCCESSFUL (shadowJar + remapJar)
• In-game 1.8.9 not available in this environment

Manual 1.8.9 checks

1. Fresh profile: Performance → 快速渲染 / Fast Render off. Enabling the module must not turn it on.
2. Homepage background picker: each card shows the matching preview.
3. Background and Music 打开文件夹: folder opens; on Windows, Explorer should come forward when possible.
4. ClickGUI Dark ↔ Light: panel/window background updates (not stuck black).
5. Expand a tall module at GUI scale 2+: settings stay clipped inside the window.

Remaining risks

• Exclusive fullscreen can still keep Explorer behind the game.
• Open-folder header buttons duplicate Prism layout math.
• ClickGUI open-scale animation: scissor does not follow the GL transform (same as before; OK at scale=1).
• Accent stays the shared indigo (Dark/Light glass, not a custom accent picker).