chore: drop redundant background repaints so surfaces inherit their parent (#27576)

* chore: drop redundant background repaints so surfaces inherit their parent

Four spots repaint the exact color their parent surface already provides
(bg-white / dark:bg-gray-900 rows inside same-colored pages and modals,
and the selectClass dark repaint inside the connection modals — the
sibling input const is already fully transparent). Visually identical in
stock light and dark; removing them lets instance theming show through
instead of leaving opaque boxes:

- .tiptap tr (app.css) — table rows in notes/editors
- Edit User Group Users tab body rows (common Modal surface)
- AddToolServerModal + AddTerminalServerModal selectClass dark repaint

The matching repaints inside the ModelUsage/UserUsage components are
not part of this change — those files were dead code and were removed
entirely in #27574.

* chore: catch remaining redundant surface repaints missed in the first pass

Same rule as the previous commit — every one of these repaints the exact
color its parent surface already provides, so removal is stock-identical
in light and dark while letting instance theming show through:

- Analytics Dashboard's inline Model Usage / User Activity row markup
  (the Analytics tab renders these tables from Dashboard.svelte itself;
  the unreferenced ModelUsage/UserUsage component files were removed
  in #27574)
- Evaluations Feedbacks + Leaderboard body rows (settings modal surface)
- admin UserList body rows (app page surface)
- chat markdown tables (MarkdownTokens): thead and body rows — unlike
  the tiptap header (gray-850 contrast, untouched), this thead painted
  the page's own color
- CitationsModal source rows (common Modal surface)
- AddConnectionModal selectClass dark repaint — third copy of the same
  const already fixed in AddToolServerModal / AddTerminalServerModal

src/app.css

@@ -685,7 +685,7 @@ html.high-contrast.dark .ProseMirror p.is-editor-empty:first-child::before {
 }
 
 .tiptap tr {
-	@apply bg-white dark:bg-gray-900 dark:border-gray-850 text-xs;
+	@apply dark:border-gray-850 text-xs;
 }
 
 .tippy-box[data-theme~='transparent'] {