feat(tui): migrate core surfaces to V2 themes (#37145)

specs/v2/tui-theme-migration.md

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+# TUI Theme V2 Migration Checklist
+
+- [x] Add semantic accent foreground and border tokens so components stop
+      reading `hue.accent[300]` directly.
+- [ ] Add paired badge or label foreground/background tokens to replace V1
+      `secondary` usages.
+- [ ] Add strong warning and error background treatments with matching readable
+      foregrounds.
+- [x] Use `text.default` for active cursors, `background.surface.offset` for
+      disabled cursors, and a lighter accent hue for focused form borders.
+- [x] Add `background.surface.offset` and `background.surface.overlay`, map
+      them from V1 panel/menu backgrounds, and use them as contextual surface
+      defaults.
+- [ ] Replace `selectedForeground` with complete V2 foreground/background pairs
+      or a V2 contrast helper that supports transparent themes.
+- [ ] Decide whether thinking opacity is fixed at `0.6` or belongs in a separate
+      presentation-token system.
+- [ ] Generate syntax styles from resolved V2 tokens, then migrate each UI
+      surface and remove the V1 proxy once no flat V1 color reads remain.