feat: add per-widget dim styling, whole widget or parens-only

[zachthedev]

Closes #438.

Adds a per-widget dim toggle alongside bold. Press (d) in the colors menu to cycle: off → whole widget → (...) spans only → off. Parens mode dims just the bracketed part of a widget, e.g. the (42%) in Context Bar output, while the rest keeps full intensity.

Area	Change
types/Widget.ts	dim: z.union([z.boolean(), z.literal('parens')]) on widget items
utils/colors.ts	applyColors gains a dim arg; applyParensDim wraps (...) spans and re-asserts bold after each span (\x1b[22m clears both intensities)
utils/renderer.ts	wired into normal and powerline paths; separators inherit dim like bold; intensity resets shared with bold
color-menu/mutations.ts	cycleWidgetDim; reset / clear-all strip dim
ColorMenu.tsx	(d) keybind, [DIM] / [DIM ()] indicator, dim shown in the widget list preview
renderer-dim.test.ts + mutations tests	applyColors dim modes, normal + powerline rendering, visible-width invariant, cycle behavior

Notes:

• No settings migration needed: the field is optional and absent means off.
• Visible text and width are unchanged, so truncation and flex math are unaffected (covered by a test).
• Zero file overlap with my other open PRs; based on current main.

[sirmalloc]

Made a few tweaks, mostly due to interaction of dim and bold widgets next to each other, preview not resetting bold properly after a dim, issues with separators in powerline mode after a dim, and the UI wrapping the dim indicator to the next line in some instances when bold was also applied. I'll publish it in the next release.