Move the per-entry {#each} key logic into a shared entryKey.ts so the
undated bucket in TimelineView can reuse it. No behavior change.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
One <section> per year with a sticky <h2> at top:4rem (REQ-006). Events render in
DTO order as pills/bands; letters render as individual cards while <= 12 (REQ-011)
or collapse to one density strip above that (REQ-012); DTO order is never re-sorted
(REQ-003). Letters carry an alternating data-side for the centered desktop axis
(REQ-004); single left column on phone (REQ-005). Derived-safe {#each} key.
Refs #779
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
