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/lib/components/AddTerminalServerModal.svelte

@@ -62,7 +62,7 @@
 	const inputClass =
 		'bg-transparent outline-hidden placeholder:text-gray-300 dark:placeholder:text-gray-700';
 	const selectClass =
-		'dark:bg-gray-900 bg-transparent pr-5 outline-hidden placeholder:text-gray-300 dark:placeholder:text-gray-700';
+		'bg-transparent pr-5 outline-hidden placeholder:text-gray-300 dark:placeholder:text-gray-700';
 
 	const stringifyJson = (value: object | null | undefined) => {
 		return JSON.stringify(value && Object.keys(value).length ? value : {}, null, 2);