fix: Windows overlay not visible (CSS url backslash + transparent compositing + follow active screen)

[touful]

Fixes #3

Summary

Three fixes for the Windows overlay not being visible on screen (verified on Windows 11, opencode 1.18.18, v1.0.2 overlay):

1. CSS url() backslash break (Windows paths) — bridge.cts built file://${spritesheetPath} from a path.resolve() result (C:\Users\...). Backslashes are escape characters in CSS url(), so the background-image value became corrupted (observed via CDP: url("file://C:Userstouful...")). Fixed by normalizing to forward slashes + file:/// prefix in:

   • preload/bridge.cts (getSpritesheetPath)
   • renderer/app.ts (onSwitchPet, same pattern)
   • main/window.ts (passes the normalized path via --spritesheet-path)

2. Fully-transparent window never composited on some Windows environments — with backgroundColor: "#00000000" (+ the opacity: 0.9999999 hack) the window content is rendered (visible via CDP Page.captureScreenshot) but never presented to the screen (screen capture shows whatever is underneath). Changing to backgroundColor: "#01000000" (alpha = 1/255, visually transparent) + opacity: 1.0 makes the pet visible with a transparent background.

3. Pet follows the active screen — the overlay positioned itself on the primary display (getPrimaryDisplay()), which on multi-monitor setups may not be where the user is working. Replaced with getActiveWorkArea() which uses the display under the cursor (falls back to primary).

Verification

Tested by unpacking the v1.0.2 release app.asar and applying the same changes to dist/:

• CDP before: background-image: url("file://C:Userstouful...") (broken URL), image never loads
• CDP after: url("file:///C:/Users/touful/.opencode-pets/overlay/resources/app/assets/pets/gutsy/spritesheet.webp"), image loads 1536x1872
• Pet visible on screen with transparent background
• Pet appears on the display where the cursor is

Notes

• winOnly is still used for thickFrame, so the opacity: 1.0 change does not orphan it.
• The 0.9999999 opacity hack becomes unnecessary once the background alpha is non-zero.