fix: tab bar icons invisible due to CSS variable isolation

Root cause: WeChat miniprogram components have style isolation by default.
Changing tab bar hardcoded #888/#666 to var(--text-secondary) broke
because --text-secondary was only defined on page{} in app.wxss and
couldn't cascade into the custom-tab-bar component.

Fix:
1. theme.js: inject BASE_VARS (--text, --text-secondary, --border, etc.)
   into themeStyle string so they're available via inline style everywhere
2. Add styleIsolation: apply-shared to all 3 component JSONs so page-level
   CSS variables cascade into components

components/calendar-heatmap/calendar-heatmap.json

@@ -1,4 +1,5 @@
 {
   "component": true,
+  "styleIsolation": "apply-shared",
   "usingComponents": {}
 }

components/progress-ring/progress-ring.json

@@ -1,4 +1,5 @@
 {
   "component": true,
+  "styleIsolation": "apply-shared",
   "usingComponents": {}
 }