polish(ui): tighter header gap, stronger row hover, readable tag chips (R13 S3)

- Channel-manager tab row: pt-4 -> pt-2, closing the too-wide gap under the
  fixed header.
- Row hover was barely visible (--card sits close to --bg in dark schemes). New
  shared --hover token (a 9% foreground mix, clear in every scheme + theme,
  auto-adapting via --fg); the .selectable recipe and the DataTable row both use
  it, so every hoverable row app-wide (tables, nav rail, menu rows) reads.
- Channel tag chips: text-3xs -> text-2xs + font-medium, so the tiny labels stay
  legible on the accent fill across all colour schemes.

Self-E2E'd: tighter gap, a table-row hover now clearly visible. gate green
(tsc/eslint-0/vitest 147/knip/prettier/build).

frontend/src/components/Channels.tsx

@@ -397,7 +397,7 @@ export default function Channels() {
     [t],
   );
   const tabs = (
-    <div className="px-4 pt-4 max-w-384 mx-auto flex items-center justify-between gap-4 flex-wrap">
+    <div className="px-4 pt-2 max-w-384 mx-auto flex items-center justify-between gap-4 flex-wrap">
       <Tabs
         tabs={viewTabs}
         active={view}