Canvas mode: annotation color palette and stroke width controls

[lidge-jun]

Source

Canvas Mode Phase 8 follow-up from devlog/_plan/0.24-canvas-mode/PHASE-8-color-palette-styles.md, FOLLOW-UP-ISSUES.md, and parent issue #22.

Goal

Let users choose annotation colors and stroke widths in Canvas Mode.

Current state

• Annotation data already stores color and strokeWidth.
• useCanvasAnnotations currently uses hardcoded defaults.
• There is no user-facing palette/stroke width UI.
• There is no validated local style preference persistence.
• Existing annotations cannot be restyled from the UI.

Scope

• Add a style popover/control in the Canvas toolbar.
• Provide preset color swatches.
• Provide stroke width choices.
• New annotations should copy the active style at creation time.
• Existing annotations must not mutate just because the active style changes.
• Persist the active style as a local UI preference only.
• Validate restored style values against allowed colors/bounds.

Out of scope

• Backend persistence for style preferences.
• Canvas-version metadata changes.
• Full layer/object inspector.
• Per-object opacity.

Acceptance criteria

• Toolbar exposes a style control.
• User can select color and stroke width before drawing.
• New pen/arrow/box/memo objects use the selected style.
• Existing annotations keep their original style when active style changes.
• Style preference survives reload.
• Escape behavior still works after style popover use.
• Tests cover style popover, persistence validation, annotation creation style copy, and no canvas-version prompt/header regression.