HRV balanced dots, dashed gap lines, dashboard widgets + drag-to-edit layout

- Green dots for balanced HRV (joining orange unbalanced / red low) on the trend
- Trend charts bridge data gaps with a dashed line instead of a blank break
- Dashboard: drop Health today; add VO2 max, small sleep, and HRV status widgets
- Dashboard: editable widget grid (react-grid-layout) with drag/resize, saved per-user

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/package-lock.json

@@ -16,6 +16,7 @@
         "react": "^18.3.1",
         "react-dom": "^18.3.1",
         "react-dropzone": "^14.2.3",
+        "react-grid-layout": "^1.5.3",
         "react-leaflet": "^4.2.1",
         "react-router-dom": "^6.23.1",
         "recharts": "^2.12.7",
@@ -2820,6 +2821,20 @@
         "react": "^18.3.1"
       }
     },
+    "node_modules/react-draggable": {
+      "version": "4.6.0",
+      "resolved": "https://registry.npmjs.org/react-draggable/-/react-draggable-4.6.0.tgz",
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+      "license": "MIT",
+      "dependencies": {
+        "clsx": "^2.1.1",
+        "prop-types": "^15.8.1"
+      },
+      "peerDependencies": {
+        "react": ">= 16.3.0",
+        "react-dom": ">= 16.3.0"
+      }
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     "node_modules/react-dropzone": {
       "version": "14.4.1",
       "resolved": "https://registry.npmjs.org/react-dropzone/-/react-dropzone-14.4.1.tgz",
@@ -2837,6 +2852,30 @@
         "react": ">= 16.8 || 18.0.0"
       }
     },
+    "node_modules/react-grid-layout": {
+      "version": "1.5.3",
+      "resolved": "https://registry.npmjs.org/react-grid-layout/-/react-grid-layout-1.5.3.tgz",
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+      "license": "MIT",
+      "dependencies": {
+        "clsx": "^2.1.1",
+        "fast-equals": "^4.0.3",
+        "prop-types": "^15.8.1",
+        "react-draggable": "^4.4.6",
+        "react-resizable": "^3.0.5",
+        "resize-observer-polyfill": "^1.5.1"
+      },
+      "peerDependencies": {
+        "react": ">= 16.3.0",
+        "react-dom": ">= 16.3.0"
+      }
+    },
+    "node_modules/react-grid-layout/node_modules/fast-equals": {
+      "version": "4.0.3",
+      "resolved": "https://registry.npmjs.org/fast-equals/-/fast-equals-4.0.3.tgz",
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+      "license": "MIT"
+    },
     "node_modules/react-is": {
       "version": "16.13.1",
       "resolved": "https://registry.npmjs.org/react-is/-/react-is-16.13.1.tgz",
@@ -2867,6 +2906,20 @@
         "node": ">=0.10.0"
       }
     },
+    "node_modules/react-resizable": {
+      "version": "3.2.0",
+      "resolved": "https://registry.npmjs.org/react-resizable/-/react-resizable-3.2.0.tgz",
+      "integrity": "sha512-3NKQ0SLZV7rs3LQHeXlOzDSRQfFrkX6TVet77/Qk03zqiZyee37b7N8/gwDJAA8UUjRz7PdWCCy49hcso45SMQ==",
+      "license": "MIT",
+      "dependencies": {
+        "prop-types": "15.x",
+        "react-draggable": "^4.5.0"
+      },
+      "peerDependencies": {
+        "react": ">= 16.3",
+        "react-dom": ">= 16.3"
+      }
+    },
     "node_modules/react-router": {
       "version": "6.30.4",
       "resolved": "https://registry.npmjs.org/react-router/-/react-router-6.30.4.tgz",
@@ -2992,6 +3045,12 @@
       "integrity": "sha512-/LLMVyas0ljjAtoYiPqYiL8VWXzUUdThrmU5+n20DZv+a+ClRoevUzw5JxU+Ieh5/c87ytoTBV9G1FiKfNJdmg==",
       "license": "MIT"
     },
+    "node_modules/resize-observer-polyfill": {
+      "version": "1.5.1",
+      "resolved": "https://registry.npmjs.org/resize-observer-polyfill/-/resize-observer-polyfill-1.5.1.tgz",
+      "integrity": "sha512-LwZrotdHOo12nQuZlHEmtuXdqGoOD0OhaxopaNFxWzInpEgaLWoVuAMbTzixuosCx2nEG58ngzW3vxdWoxIgdg==",
+      "license": "MIT"
+    },
     "node_modules/resolve": {
       "version": "1.22.12",
       "resolved": "https://registry.npmjs.org/resolve/-/resolve-1.22.12.tgz",