fix: Restore the stock limit bar colors as the default after #20

PR #20 made the session / weekly limit bar colors configurable, but shipped
fixed green and blue as the new default. That repainted every existing user's
menu bar on upgrade, dropped the Usage dashboard's orange middle tier, and
moved the red warning from "above 90% used" to "at or above 70% used".

The colors are stored as hex strings, which cannot express `.primary` — the
dynamic color that let the menu-bar fill follow a light or dark menu bar — nor
a three-tier gradient. So restoring the old default needed a way to say
"no custom color", not just different constants.

Add an opt-in `customizesLimitBarColors` switch, off by default. While off,
`stockColor(utilization:context:)` reproduces the pre-#20 palettes verbatim:

- Menu-bar strip: monochrome `.primary`, red only when utilization > 90.
- Usage dashboard: red at >= 90, orange at >= 60, green below.

`LimitBarContext` distinguishes the two, since they never shared a palette.
The pickers and the threshold slider collapse while the switch is off, which
also keeps the module list and its preview visible without scrolling — the
new section had pushed them out of the fixed-height Settings window.

The threshold slider gained a caption: it sets remaining quota, not used, so
the default 30% means the bars turn at 70% used.

Verified on a signed Debug build against real accounts: a fresh launch shows
the monochrome menu-bar bars and the green / orange / red dashboard again, and
enabling the switch still applies the custom colors.