Make the brand purple the accent color and exercise Done check

Populate the previously-empty AccentColor asset (iOS + watch) with the
logo purple — a deep shade in light mode, brightened for dark mode and
the watch's black background. The exercise Done check now uses that
accent color and the in-progress indicator reads as a neutral gray, on
both iPhone and Apple Watch.

CLAUDE.md

@@ -92,9 +92,10 @@ Seeded on demand, never automatically — an empty cache at launch is indistingu
 
 `CHANGELOG.md` is bundled and shown in-app via IndieAbout, so write it for **end users**, not developers. When you make user-facing changes, add entries following these rules:
 
+- No preamble: the file starts directly with the first month heading — no title or intro line (e.g. **Changelog** / "All notable changes…").
 - Group entries by month, newest month first; a version milestone (e.g. **2.0**) may prefix the entry it applies to.
 - Write each entry as its own blank-line-separated paragraph — **no bullet or dash markers**, because the Apple inline-Markdown subset IndieAbout renders doesn't style them (see the `indie-about` skill for the supported subset).
 - Derive entries from the git log, but rewrite (don't copy) each into a concise, end-user-understandable description of the crux of the change.
-- Include only changes significant to an end user; skip internal / tooling / refactor-only commits.
+- Include only changes significant to an end user; skip internal / tooling / refactor-only commits. In particular, never log changes to the changelog itself (its preamble, formatting, or how it's derived) — these have zero end-user value.
 - When one commit holds several user-facing changes, split them into separate paragraphs.
 - When several commits address the same user-facing change, collapse them into one paragraph.