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/chat/Messages/Citations/CitationsModal.svelte

@@ -60,7 +60,7 @@
 				{#each citations as citation, idx}
 					<button
 						id={`source-${id}-${idx + 1}`}
-						class="no-toggle outline-hidden flex dark:text-gray-300 bg-white dark:bg-gray-900 rounded-xl gap-1.5 items-center"
+						class="no-toggle outline-hidden flex dark:text-gray-300 rounded-xl gap-1.5 items-center"
 						on:click={() => {
 							showCitationModal = true;
 							selectedCitation = citation;

src/lib/components/chat/Messages/Markdown/MarkdownTokens.svelte

@@ -193,9 +193,7 @@
 					class=" w-full text-sm text-start text-gray-500 dark:text-gray-400 max-w-full rounded-xl"
 					dir="auto"
 				>
-					<thead
-						class="text-xs text-gray-700 uppercase bg-white dark:bg-gray-900 dark:text-gray-400 border-none"
-					>
+					<thead class="text-xs text-gray-700 uppercase dark:text-gray-400 border-none">
 						<tr class="">
 							{#each token.header as header, headerIdx}
 								<th
@@ -220,7 +218,7 @@
 					</thead>
 					<tbody>
 						{#each token.rows as row, rowIdx}
-							<tr class="bg-white dark:bg-gray-900 text-xs">
+							<tr class="text-xs">
 								{#each row ?? [] as cell, cellIdx}
 									<td
 										class="px-3! py-2! text-gray-900 dark:text-white w-max {token.rows.length -