feat(image-editor): wire Dodge tool — paint a stroke that brightens pixels

[lyfuci]

Summary

Promotes the Dodge (O) tool from stub to functional. Drag on the canvas
to paint a stroke that brightens the underlying pixels. Same idea as
PS's Dodge: each layer represents one stroke, layer opacity controls
intensity, and repeated strokes build up.

Implementation

• `BrushShape` gains an optional `mode: 'dodge' | 'burn'` field. When
  set, `drawBrush` overrides the stroke color (white for dodge, black
  for burn) and the composite operation (`'lighter'` / `'multiply'`).
  Per-layer save/restore around `drawShape` already isolates these
  changes from siblings.
• `Canvas.tsx` adds a `tool === 'dodge'` branch that creates a brush
  layer with `mode: 'dodge'` and default opacity 30 so a single stroke
  gives a soft brighten — repeat to build up.
• `OptionsBar` shares the brush variant (stroke width slider) and adds
  a dodge-specific hint.
• `LayersPanel` labels dodge / burn brushes distinctly.
• The `'burn'` mode is wired in the data model + drawing path + label
  table but no UI tool selects it yet — folded in so a future Burn-tool
  PR is a one-line palette change.

Removes `dodge` from `STUB_TOOLS`.

Branched from `main` (independent).

Test plan

• Press O — dodge tool active, options bar shows stroke width + dodge hint.
• Drag across the image — area visibly brightens (default 30 % layer opacity).
• Adjust the new layer's opacity in the right panel — dodge gets stronger / weaker.
• Repeat strokes — brightness builds up.
• Cmd+Z undoes; Cmd+Shift+Z redoes.
• Save .json project, reload — dodge stroke persists with mode preserved.
• Export PNG — dodge baked at source resolution.

🤖 Generated with Claude Code