tui,dashboard: simplify Stats to single Total tokens row

v0.21.8 was about to ship with a three-row Stats section (New work,
Reprocessed, Total input). User feedback before release: the three-way
split needs more user-facing explanation than the value it delivers and
would generate support questions.

Hide New work and Reprocessed rows for now. Rename Total input ->
Total tokens. Keep the underlying snapshot fields (newWorkTokens,
totalInputTokens) and session_meta columns populated by the runtime
so a later release can re-introduce the categorization once we have a
cleaner UX for it.

UI-only change. No runtime, schema, or RPC contract changes.

- packages/plugin/src/tui/slots/sidebar-content.tsx: collapse three
  StatRows into one Total tokens row; guard switched from
  newWorkTokens != null to totalInputTokens != null
- packages/dashboard/src/components/SessionViewer/SessionViewer.tsx:
  collapse three Meta-tab rows into one Total tokens row