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

[JoeJoeflyn]

The font-size input, increment, and decrement buttons were disabled unless a text node was selected. This blocked 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: The disabled guard now checks hasNodes instead of selectedNode, so the controls stay enabled whenever the page has text boxes. Commits route through applyStyleToSelected when a node is selected and applyStyleToAll otherwise, preserving the existing per-node behavior. A local state buffer (globalFontSizeInput) holds the input value 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.

Fixes: #649

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

screenrecording-2026-07-27_00-50-21.mp4

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