Make board switching immediate and fix image exports

[ojowwalker77]

What changed

• replaces the hover board picker with horizontally scrollable, always-visible board pills and a dedicated new-board button
• keeps rename and delete actions available from each board pill
• resolves saved attachment filenames through AssetStore during PNG export, while preserving legacy absolute-path support
• adds render coverage for stored images and whole-board exports with distant image cards
• rolls the accumulated release notes into 1.4.7, intentionally skipping a separate 1.4.6 changelog section

Why

Board switching previously hid every non-current board behind a hover menu, adding friction to a frequent action. Image exports also treated persisted relative attachment names as absolute paths, producing placeholders instead of the actual image pixels.

User impact

Boards are now directly accessible from the canvas chrome, and Copy PNG / Save PNG correctly include saved image cards across the full board bounds.

Validation

• swift test — 169 tests passed
• git diff --check