feat(chart): keyboard navigation for chart panning

[terrylica]

Adds keyboard-driven chart panning so users can navigate history without touching the mouse or trackpad.

Key bindings:
←/→ scroll 10 bars left/right
Shift+←/→ scroll 50 bars (fast)
PageUp/Down scroll one full visible window
Home jump to latest bar (reset pan)

macOS equivalents: Fn+↑ = PageUp, Fn+↓ = PageDown, Fn+← = Home

Navigation works whenever the app window is focused; the cursor does not need to be inside the chart canvas.

Implementation

All pan logic lives in a new src/chart/keyboard_nav.rs module.

keyboard_nav_msg() is added as a default method on the Chart trait (using the already-required state() accessor), so every current and future chart type automatically gets keyboard navigation with no per-type boilerplate.

Minimal changes to existing files:

• src/chart.rs mod declaration + default trait method +
  delegate _ => arm in canvas_interaction;
  cursor_position? guard moved inside Shift arm
  (ruler tool needs it, nav keys do not)
• src/screen/dashboard.rs ChartKeyNav message + handler
• src/screen/dashboard/pane.rs apply_keyboard_nav() + Chart import
• src/main.rs extend keyboard::listen() to forward nav keys
  app-wide (canvas delivery requires cursor focus)

Tested on: Apple M3 Max, macOS 15.7.5 Sequoia (Build 24G607)