fix: keep focused properties in sync with selection

[BasicallyPolaris]

Closes #48

Summary

• model focused property editing as a transient Zustand edit session instead of pausing and manually rewriting undo history
• derive controlled input values and canvas/ruler previews from the active session, with explicit commit and Escape discard behavior
• centralize persistent mutations so active edits commit before document actions
• use preview geometry for drag, resize, marquee, drawing, and context-menu interactions
• keep dirty tracking and export persistence aware of active previews

Verification

• bun run test — 305 tests passed
• bun run build — passed
• two-axis standards/spec review — no remaining findings