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/utils/diff.ts

@@ -1,14 +1,10 @@
 /**
  * Line diff — one copy, for every surface that shows what changed.
  *
- * WHY THIS FILE EXISTS. The same LCS walk was written out three times:
- * privately in `useAssist.ts`, and again inside `HistoryPanel.vue` and
- * `VersionHistorySection.vue`. The three were character-identical apart from
- * quote style — nobody had diverged them on purpose, they were simply copied
- * because `computeDiff` was never exported. Milestone 323 needed a fourth
- * consumer (a rule's edit history), and a fourth copy is the cost #3207
- * records: a fix or an improvement now has to be found in N places by someone
- * who does not know N.
+ * WHY THIS FILE EXISTS. The one LCS diff walk, shared by `useAssist.ts`,
+ * `HistoryPanel.vue`, `VersionHistorySection.vue` and the rule edit history.
+ * A copy per consumer means a fix has to be found in N places by someone who
+ * does not know N.
  */
 
 export interface DiffLine {