feat(menu-bar): add monochrome icon mode with template rendering

[jeanfw]

Summary

Adds an opt-in monochrome mode for the menu bar icon so it matches the system tray's white-on-dark / black-on-light aesthetic instead of standing out with green/orange/red status colours.

• New Use Colored Status Icon toggle in Settings → General (default off, monochrome).
• When off, the icon is rendered as a template image (NSImage.isTemplate = true) so macOS tints it to match the menu bar.
• When on, the original status palette (green → yellow → orange → red, plus purple for the weekly bar in DualBar) is preserved exactly — pixel-perfect against existing snapshots.
• Colour resolution is centralised in a new MenuBarIconColors helper and threaded through every IconStyle plus the renderer and the cache key.
• AppSettings stays backwards compatible: a saved settings blob without use_colored_icon decodes with the new field defaulted to off (custom init(from:) falling back to .default).

Why monochrome by default

Most macOS menu bar apps (system, third-party, Apple's own) render their tray icon as a template image so it inherits the menu bar tint. ClaudeMeter's coloured icon sticks out visually next to them. Making monochrome the default brings it in line with the system aesthetic; users who liked the at-a-glance colour-coded status can flip the toggle back on.

Preview

The existing #Preview(\"All Styles\") block still shows the coloured layout. A new #Preview(\"Monochrome\") block renders all six styles in the new mode.

Test plan

• Open Settings → General; verify the new toggle appears with the description text.
• With toggle off (default): icon adapts to dark/light menu bar — appears white in dark mode, black in light mode.
• With toggle on: green at low usage, orange at warning, red at critical, gray when stale (as before).
• Cycle through all six icon styles in both modes.
• Existing MenuBarIconRendererTests and MenuBarIconSnapshotTests still pass (colour mode is the default, matches old behaviour).
• Upgrading from an older build: settings load without error, useColoredIcon initialises to off.

🤖 Generated with Claude Code