feat: optional anchor for iPadOS/macOS share popovers

[karlingen]

Implements the optional anchor field requested in #11 so that the share popover on iPadOS/macOS can anchor to a source rectangle instead of always centering on the window.

Changes

• Added anchor?: { x, y, width, height } to ShareData (guest-js/index.ts) and ShareOptions (src/models.rs).
• iOS/iPadOS (ios/Sources/SharePlugin.swift): caches the WKWebView from load(webview:), then sets popoverController.sourceView and popoverController.sourceRect from the anchor when provided, falling back to the previous centered behavior.
• macOS (src/platform/macos.rs): converts the anchor rect from web-viewport (top-left origin) coordinates to AppKit (bottom-left origin) coordinates and passes it to showRelativeToRect_ofView_preferredEdge, falling back to the view center when no anchor is provided.
• Android (android/src/main/java/SharePlugin.kt): added Anchor/ShareOptions.anchor models so deserialization stays compatible; the anchor is otherwise ignored.
• Updated README with a button-anchor example and the Rust ShareOptions snippet.
• Added a JS regression test verifying the anchor is forwarded to the Rust command.

Verification

• cargo +1.89.0-aarch64-apple-darwin test — 11 passed
• cargo +1.89.0-aarch64-apple-darwin fmt --check — clean
• cd ios && VNIDROP_SHARE_USE_TAURI_STUB=1 swift test — 5 passed
• npm run build — dist-js and api-iife.js regenerated
• npm run test — 8 passed
• npm run test:types — clean

Closes #11

[AbassHammed]

This looks good, need to test this locally then i can merge. Thanks though @karlingen