feat(fitness/measure): consolidate entries by day + richer past-measurements summary

- Server POST now upserts by (user, calendar day). Non-conflicting
  fields merge silently; real overwrites (new non-empty value ≠
  stored value) return 409 with the conflict list. Client retries
  with `?overwrite=1` after a confirm dialog naming each field and
  its old→new value. Null/empty payload fields are skipped, so
  logging a body-fat entry on a day that already has weight merges
  cleanly without flagging a phantom weight conflict.
- `summaryParts` in the history now includes a body-parts count,
  e.g. "86 kg · 0.1% bf · 5 body parts" or "5 body parts" instead
  of the flat "Body measurements only" fallback. Pluralised in EN
  and DE.
- Inline quick-edit: "Full edit →" text replaced by a dashed primary
  pill `Pencil · Edit all fields · ChevronRight`, inlined with the
  X / ✓ action buttons on the same row. The label collapses to
  icons only at ≤480px so the three controls stay on one line.
- Quick-edit date input swapped from native `<input type="date">`
  to the site's `DatePicker` component.
- New i18n: `overwrite_title`, `overwrite_message`, `overwrite_confirm`.
- TODO.md marks features #2 and #3 done. CLAUDE.md carries a
  policy note (no AI-attribution trailers on commits).

TODO.md

@@ -1,5 +1,13 @@
 # TODO
 
+## Features
+[x] on /fitness/measure, fill "Past measurements" in SSR only for the last 10 measurements. anything further should be fetched client side on mount to decreae initial page load time. use a "show more" button and paginate measurments.
+[x] on /fitness/measure (resp. their associated logging API routes), consolidate measurements by day. If we want to log another measurement, overwriting an old one, show a warning to indicate this. disparate measurements (e.g., weight and bodyfat) should not show this warning but simply be merged into one log entry for that day.
+[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.
+[ ] 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
+[ ] 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?
+
 ## Refactor Recipe Search Component
 
 Refactor `src/lib/components/Search.svelte` to use the new `SearchInput.svelte` component for the visual input part. This will: