Add floating window size presets and custom dimensions

[FaustUser]

This PR improves the floating window size settings and addresses issue #42.
Previously, the floating window had a fixed size. This change adds both preset sizes and manual dimension controls.

What changed

• added floating window size presets: Small, Default, Large
• added optional custom size mode with manual Width and Height inputs
• made the Electron floating window resize immediately when settings change
• scaled the floating window UI content based on the effective height so custom sizes remain usable
• added validation/clamping for manual inputs
• if a value below the minimum is entered, it is reset automatically to the minimum
• added a tooltip explaining that the first field is width, the second is height, and showing the minimum allowed values

Behavior

• preset mode keeps the simple predefined size options
• custom mode allows manual sizing
• custom dimensions are persisted in app config
• the selected size is preserved after hiding/showing the floating window and after app restart

Validation

• ran npm run typecheck
• manually tested:
  • preset switching
  • custom width/height updates
  • automatic reset when entering values below the minimum
  • persistence after reopening the floating window