fix(desktop): align context usage and compaction status

[Git-on-my-level]

Summary

• keep desktop context usage popup internals consistent by scaling rough category buckets to the provider-measured prompt total when real usage is available
• re-baseline the status bar from the live session.context_breakdown snapshot when the popup opens, so the footer and popup stop showing stale/different context totals
• add an explicit compaction-complete status edge and clear the desktop "Summarizing thread" indicator as soon as compaction finishes instead of leaving it latched for the rest of the turn

Issue

The desktop app could show multiple incompatible context numbers at the same time. For example, the footer might show 128.2k/272.0k (47%), while the context popup showed ~241.4k/272.0k (89%), and the popup's category rows could sum to a third value or even show Conversation larger than the header total.

Root causes:

1. the popup header used provider-measured last_prompt_tokens, but its category rows were independent rough estimates;
2. the footer used the last pushed usage event while the popup fetched a fresh live breakdown;
3. auto-compaction only emitted a start event, so the desktop treated "Summarizing thread" as a turn-long latch rather than active summarization work.

Test Plan

• uv run --with pytest python -m pytest tests/agent/test_context_breakdown.py tests/tui_gateway/test_compaction_status.py tests/test_tui_gateway_server.py::test_get_usage_reports_real_current_occupancy tests/test_tui_gateway_server.py::test_get_usage_clamps_post_compression_sentinel -q -o 'addopts='
• npm run test:ui -- src/app/shell/context-usage-panel.test.tsx src/store/compaction.test.ts
• npm run typecheck
• npx eslint src/app/shell/context-usage-panel.test.tsx src/app/shell/context-usage-panel.tsx src/app/shell/hooks/use-statusbar-items.tsx src/app/session/hooks/use-message-stream/gateway-event.ts
• git diff --check