feat(fitness): rename Measure route to Check-in / Erfassung (NotebookPen icon)

Route slugs and nav label rename only — storage, API endpoints
(`/api/fitness/measurements`), and the `BodyMeasurement` Mongo model
keep their technical names.

- `/fitness/measure` → `/fitness/check-in` (EN)
- `/fitness/messen` → `/fitness/erfassung` (DE)
- Folder `[measure=fitnessMeasure]` → `[checkin=fitnessCheckIn]`
  (git rename; history preserved).
- Param matcher `fitnessMeasure.ts` → `fitnessCheckIn.ts`, accepts
  `check-in` / `erfassung`.
- `fitnessSlugs(lang).measure` and `fitnessLabels(lang).measure` code
  keys are unchanged — value returns "check-in"/"erfassung" and
  "Check-in"/"Erfassung" respectively, so no call site needs touching.
- slugMap language-detection updated to `erfassung ↔ check-in`.
- Service-worker cache list + the layout regex that gates the wider
  content width now reference the new slugs.
- Nav icon swapped from `Ruler` to `NotebookPen` — reads as "logging
  entries" and spans weight / composition / period better.

Bookmarks on the old URLs will 404; no redirect added.

TODO.md

@@ -6,7 +6,7 @@
 [x] on /fitness/measure in the past measurments tab, show more than "Body measurements only" if we don't have Bodyweight logged. we can be a bit more elaborate in our syntax here tbh.
 [x] add a button on /fitness/measure/body-parts for each measurement directly below to say "Same value", instead of having to hit +, then - to lock in same number
 [x] BF graph (with trend line like weight graph) on /fitness/stats page. Emphasize relative changes, not absolute numbers in design (as we cannot trust those) (e.g., use start day of overview as 0% and then show +/- x % on the graph)
-[ ] Workshop better names than "Measure" for the /fitness/measure route. It's about body data points (i.e., non-food related). What's a better, short name than "Measure" to capture the logging of weight, body composition, body part measurements, and period tracking?
+[x] Workshop better names than "Measure" for the /fitness/measure route. It's about body data points (i.e., non-food related). What's a better, short name than "Measure" to capture the logging of weight, body composition, body part measurements, and period tracking?
 [ ] on /fitness/stats/histoy/<part> for body measurement graphs, make the range reasonable. e.g., if we have 1 cm change, do not fill the entire y-height with 1 cm. Use reasonable padding for low ranges (i think we do something like htis already on the weight graph?)
 
 ## Refactor Recipe Search Component