feat: fold dasha periods and redraw the western wheel

The dasha tab leads with a now card and a duration bar, and keeps sub-periods inside the current details element. A zero-year Chara row stays grey, shows 0 年, and cannot expand. The tropical wheel uses a 520 view box, element bands, separated planet labels, and aspect lines that stay visible while a selected body is highlighted.

frontend/src/lib/chart-view-contract.ts

@@ -213,6 +213,9 @@ export const chartViewOkSchema = z.object({
     planets: z.array(chartViewPlanetRowSchema).min(1),
   }),
   dasha: z.object({
+    // The day the current flags were computed against. The dasha tab uses it
+    // for the "现在" card and the timeline cursor. It is not an engine field.
+    asOf: isoDate,
     vimshottari: chartViewDashaTrackSchema,
     chara: chartViewDashaTrackSchema,
     dualTrackNote: z.string().min(1),

frontend/src/lib/chart-view-mapper.ts

@@ -695,6 +695,7 @@ export function buildChartView(bundle: ChartViewEngineBundle): ChartViewOk {
       planets: planetRows(chart, varga),
     },
     dasha: {
+      asOf: isoDay(asOf),
       vimshottari,
       chara: charaMapped,
       dualTrackNote: DASHA_DUAL_TRACK_NOTE,