docs(6208): the remaining frontend comments drop milestone, step and rule citations

Components, stores, api clients, router, types and shared CSS now state what
each piece is and why, without the milestone that introduced it. diff.ts
names its consumers instead of the copy history; stores/notes and tasks state
their current scope instead of the cleanup that set it; theme.css states the
token practice instead of the sweep that established it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/assets/theme.css

@@ -208,11 +208,8 @@
  */
 
 /* ==========================================================================
-   The compatibility-alias block that lived here is GONE (#2533). It let ~55
-   components keep their historical --color-* names while theme.css was
-   repointed at the design system; the rename sweep it promised ran on
-   2026-08-08 and every component now references --fs-* directly. Do not
-   reintroduce app-local alias names — the design system's tokens are the
+   Every component references the design system's --fs-* tokens directly.
+   Do not introduce app-local alias names — the design system's tokens are the
    vocabulary, and check_snippets_against_design_system can only see through
    names the system actually declares.
    ========================================================================== */