refactor(design-system): migrate fg-* utilities to text-* and remove namespace

The design system carried two parallel namespaces for foreground colors:
text-* (canonical, ~2,000 uses) and fg-* (32 uses). Most fg-* tokens
were 1:1 duplicates of a text-* counterpart. fg-gray was nearly
identical to text-secondary, with a one-step shade difference in dark
mode.

This PR migrates all 32 usages to their text-* equivalents and removes
the fg-* block from the design tokens. Closes #1606.

Mapping:
- fg-inverse  -> text-inverse  (20 usages, identical light/dark values)
- fg-gray     -> text-secondary (7 usages; light values match, dark is
                                 one step lighter: gray-300 vs gray-400)
- fg-primary  -> text-primary  (3 usages, identical values)
- fg-subdued  -> text-subdued  (2 usages, identical values)

The four other fg-* tokens (fg-contrast, fg-primary-variant,
fg-secondary, fg-secondary-variant) had zero usages despite being
defined; they are removed without replacement.

JSON / build:
- design/tokens/sure.tokens.json: $version 1.0.0 -> 2.0.0 (breaking
  schema change per the policy added in #1620). 8 fg-* token
  definitions removed.
- button-bg-ghost-hover's dark value still references "fg-inverse"
  internally; rewritten to "bg-gray-800 text-inverse" so the cleanup
  doesn't break that utility.
- _generated.css regenerated. 42 utility blocks now (was 50).

Lookbook tokens preview:
- The Text & foregrounds section dropped its split between text-*
  (canonical) and fg-* (legacy). Now a single section listing the
  five text-* utilities. The "(legacy)" framing is gone since there's
  no legacy left.

README:
- design/tokens/README.md's button-bg-ghost-hover edge-case example
  updated to reflect the new "bg-gray-800 text-inverse" dark value.

Visual review needed in dark mode:
- Anywhere icons use the application_helper#icon helper with
  color: "default" (most icons in the app). The default class moved
  from fg-gray (gray-400 dark) to text-secondary (gray-300 dark), so
  default-color icons render slightly lighter in dark mode.
- DS::Buttonish icons in secondary buttons (same shade shift).
- DS::Link icons (same).
- Time series chart axes (same).
- All tooltips, account add flow, settings hostings buttons,
  invitations, AI consent, family export, danger-zone buttons --
  these used fg-inverse, which is identical to text-inverse, so no
  visual change expected.

app/javascript/controllers/time_series_chart_controller.js

@@ -110,7 +110,7 @@ export default class extends Controller {
       .attr("cx", this._d3InitialContainerWidth / 2)
       .attr("cy", this._d3InitialContainerHeight / 2)
       .attr("r", 4)
-      .attr("class", "fg-subdued")
+      .attr("class", "text-subdued")
       .style("fill", "currentColor");
   }
 
@@ -220,7 +220,7 @@ export default class extends Controller {
     // Style ticks
     this._d3Group
       .selectAll(".tick text")
-      .attr("class", "fg-gray")
+      .attr("class", "text-secondary")
       .style("font-size", "12px")
       .style("font-weight", "500")
       .attr("text-anchor", "middle")
@@ -334,7 +334,7 @@ export default class extends Controller {
         // Guideline
         this._d3Group
           .append("line")
-          .attr("class", "guideline fg-subdued")
+          .attr("class", "guideline text-subdued")
           .attr("x1", this._d3XScale(d.date))
           .attr("y1", 0)
           .attr("x2", this._d3XScale(d.date))