feat(visualization): add label size slider for floating labels

Adds a "Label Size" slider in the Label Settings panel that scales
floating CSS2D labels (name, metric value, and "+N more" badge) by a
single multiplier between 0.75x and 2.5x (default 1x). The setting is
persisted with scenarios and resets via the existing reset button.

Also fixes a pre-existing bug in loadInitialFile.service.ts where the
"amountOfEdgePreviews" key dispatched setAmountOfTopLabels instead of
setAmountOfEdgePreviews when restoring saved state.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>