fix(dashboard): open the body fat page from the body fat tile

The body fat tile was the one strip tile that led nowhere. Its href map
said there was no dedicated sub-page, which was true: body fat percentage
was tiled and charted on the dashboard but never got the Insights page
every body-composition sibling has.

Add /insights/body-fat on the generic metric-page scaffold (body group,
tab-strip pill, seven locales, manual-entry empty state) and link the
tile to it.

Move the tile href map out of the page into tile-destinations.ts, keyed by
the tile-capable widget id and typed against the sub-page slug union, so
a missing or non-existent destination fails to compile. The link passes
the destination's insights-page surface gate, so it never points at a
page a switched-off module hides.

A guard derives the tile set from TILE_CAPABLE_WIDGET_IDS and from the
ids the dashboard actually pushes into its strip, and fails when a tile's
metric has a sub-page the tile does not open, when a destination has no
route, or when a tile links to a page owned by another module. Tiles that
deliberately lead nowhere need a written reason; there are none today.

The reverse-lookup test pinned BODY_FAT as having no page; it now
resolves to body-fat.

Refs #1084

docs/api/openapi.yaml

@@ -25902,7 +25902,7 @@ components:
               - order
         tiles:
           minItems: 1
-          maxItems: 62
+          maxItems: 63
           type: array
           items:
             type: object
@@ -25919,6 +25919,7 @@ components:
                   - pain
                   - weight
                   - bmi
+                  - body-fat
                   - body-water
                   - bone-mass
                   - fat-free-mass
@@ -31264,7 +31265,7 @@ components:
                 additionalProperties: false
             tiles:
               minItems: 1
-              maxItems: 62
+              maxItems: 63
               type: array
               items:
                 type: object
@@ -31281,6 +31282,7 @@ components:
                       - pain
                       - weight
                       - bmi
+                      - body-fat
                       - body-water
                       - bone-mass
                       - fat-free-mass
@@ -51976,7 +51978,7 @@ components:
             additionalProperties: false
         tiles:
           minItems: 1
-          maxItems: 62
+          maxItems: 63
           type: array
           items:
             type: object
@@ -51993,6 +51995,7 @@ components:
                   - pain
                   - weight
                   - bmi
+                  - body-fat
                   - body-water
                   - bone-mass
                   - fat-free-mass