fix(a11y): make assetGridItemSemanticIndex 1-based to match previous behavior

[luomo-pro]

What

DefaultAssetPickerBuilderDelegate.assetGridItemSemanticIndex currently returns:

return index - prependItems.length;

which produces a 0-based index. As a result, screen readers (VoiceOver / TalkBack) announce assets as "Image 0, Image 1, ..." which is unintuitive and inconsistent with the 1-based indices used by previous versions of this package (e.g. "Image 1, Image 2, ...").

This PR adds + 1 to restore the 1-based announced index.

Why

The 1-based behavior was the long-standing convention in older versions of wechat_assets_picker, where the equivalent function returned index + 1 (no prepend) or index (with prepend). Both branches yielded a 1-based announced index for the first asset.

When this method was refactored to support the new multi-prepend specialItems API, the + 1 offset was inadvertently dropped. The fix restores the original semantics in a single line, with no behavioral change to non-accessibility code paths.

Behavior comparison

With one prepend item present:

Grid index	Previous behavior	Current main	This PR
0 (prepend tile)	n/a	n/a	n/a
1 (1st asset)	1	0	1
2 (2nd asset)	2	1	2

Without prepend items:

Grid index	Previous behavior	Current main	This PR
0 (1st asset)	1	0	1
1 (2nd asset)	2	1	2

Test plan

• Open a picker (with or without prepend special items).
• Enable VoiceOver (iOS) or TalkBack (Android).
• Swipe through the grid and confirm assets are announced as "Image 1, Image 2, ..." rather than "Image 0, Image 1, ...".

Notes

• Single-line change in lib/src/delegates/asset_picker_builder_delegate.dart.
• CHANGELOG updated under the Unreleased section.