feat(line): smart axis snap + quick-orientation picker

[u8array]

Endpoint editing:

• onDragMove and onDragEnd both project the cursor onto a constrained axis so the line visually stays on a rail during the drag instead of only snapping at release. Length matches the axial component of the drag (Figma / Sketch convention) rather than the diagonal Euclidean distance.
• Default behaviour: Figma-style auto-snap to the nearest 45° step when the raw angle is within ±5°, otherwise free.
• Shift always forces an explicit 45°-step constraint.

Quick-orientation picker in the Properties panel:

• Four buttons (—, |, /, ) rendered as inline 12×12 SVGs for crisp, font-independent icons.
• Buttons pick the candidate angle closest to the line's current angle so the line keeps its rough direction. Repeated click on the active orientation flips the line 180° (e.g. 0° ↔ 180°).
• View-rotation aware: candidates are expressed as on-screen angles, then converted to label-space by subtracting the current view rotation. Clicking '—' always yields a visually horizontal line.
• Length is preserved (no projection collapse when rotating).

[gemini-code-assist]

Code Review

This pull request introduces a centralized line constraint system with free, shift, and auto-snap modes, refactoring the LineObject component to use a new projection utility. It also adds an orientation picker to the line properties panel that remains accurate under canvas rotation and includes updated translations for multiple locales. Feedback suggests enhancing the orientation picker's toggle logic by using angular distance instead of strict equality to prevent issues with angle normalization.