fix: unlock font-size field for global editing without selection - #876

The font-size input, increment, and decrement buttons were disabled
unless a text node was selected, blocking the global workflow: typing
a value with nothing selected should apply to every text box on the
page, and clearing it should reset all boxes to auto.

What changed:
- Switch the disabled guard from !selectedNode to !hasNodes so the
  controls stay enabled whenever the page has text boxes.
- Route commits through applyStyleToSelected when a node is selected
  and applyStyleToAll otherwise, so the existing per-node behavior is
  preserved.
- Buffer the input in local state (globalFontSizeInput) while in
  global scope, otherwise the controlled value resets to '' on each
  keystroke because currentFontSize stays undefined with no selection.

User-visible behavior differences:
- With no text node selected, the font-size field is now editable
  (previously greyed out). Typing a number applies it to every text
  box on the page; clearing the field resets all boxes to auto.
- Selecting a single text box still applies changes only to that
  node, unchanged.

How I verified:
- bun run format and bun run test:ui (158/158 passed, including new
  tests covering the global behavior).
- Manually confirmed against a running app: with no text node
  selected, typing 24 sets fontSize=24 on all 8 text boxes; clearing
  the field reverts all to auto (null) in the backend scene.

Closes mayocream#649.

This patch was made with AI assistance (Devin/Claude). The code was
reviewed and tested before submission.