fix(charts): restyle hover tooltips with soft shadow + larger radius

Match the elevation pattern of DS::Select dropdown
(shadow-lg + shadow-border-xs) and increase radius (rounded-lg →
rounded-xl) + padding (p-2 → p-3) for better breathing. Drops the
hard border in favour of the soft drop shadow.

time_series_chart_controller:
- p-2 → p-3, rounded-lg → rounded-xl
- remove border border-secondary
- add shadow-lg shadow-border-xs (theme-aware drop + border-edge)
- add explicit text-primary (was missing per #2011's drift note)
- add z-50 (matches goal_projection + sankey controllers)

sankey_chart_controller:
- bg-gray-700 text-white → bg-container text-primary (theme-aware;
  was broken in light mode after dark-bg flip)
- p-2 rounded → p-3 rounded-xl
- add shadow-lg shadow-border-xs
- add font-sans for consistency with the other chart tooltips
- add privacy-sensitive class (was missing — sankey money values
  were rendered in the clear with privacy mode on)

DS::Tooltip (icon-trigger help, bg-inverse) is intentionally a
different primitive and is not touched.

Refs #2011 — className consolidation into a shared module is tracked
separately and intentionally not closed by this PR; the new string
applies to two of the three call sites today (time_series, sankey).
The third (goal_projection_chart_controller) lives on the
feat/goals-v2-architecture branch and will adopt the same string when
goals v2 merges.

app/javascript/controllers/sankey_chart_controller.js

@@ -511,7 +511,7 @@ export default class extends Controller {
       .append("div")
       .attr(
         "class",
-        "bg-gray-700 text-white text-sm p-2 rounded pointer-events-none absolute z-50 top-0",
+        "bg-container text-primary text-sm font-sans absolute p-3 rounded-xl shadow-lg shadow-border-xs pointer-events-none z-50 top-0 privacy-sensitive",
       )
       .style("opacity", 0)
       .style("pointer-events", "none");

app/javascript/controllers/time_series_chart_controller.js

@@ -289,7 +289,7 @@ export default class extends Controller {
       .append("div")
       .attr(
         "class",
-        "bg-container text-sm font-sans absolute p-2 border border-secondary rounded-lg pointer-events-none opacity-0 top-0 privacy-sensitive",
+        "bg-container text-primary text-sm font-sans absolute p-3 rounded-xl shadow-lg shadow-border-xs pointer-events-none opacity-0 top-0 z-50 privacy-sensitive",
       );
   }