feat(usage): complete session usage report

[limityan]

Summary

This PR completes the approved P1 scope for the current-session /usage report and tightens the remaining Desktop analysis surface work into a closed product boundary.

• Adds richer session usage aggregation across persisted runtime facts, token/model/tool/file/error summaries, workspace identity, and privacy metadata.
• Hardens the Desktop usage report card and detail panel with localized copy, partial-data explanations, metadata copy controls, path redaction preference, file diff actions, and representative transcript anchors.
• Fixes usage-detail navigation for grouped or auto-collapsed tool cards by expanding the relevant explore group and focusing the real tool item anchor.
• Updates the design document to describe the closed product scope: current session only, durable model-invisible local report, Chat-bottom entry point, no live metrics, no cross-session analytics, and no billing/cost reporting.
• Removes residual cost_estimates coverage markers so the report stays token-focused and does not imply a billing feature.

Product Boundary

The report is intentionally an observability projection, not a runtime control plane. It reports recorded current-session facts and coverage gaps, but it does not change scheduling, retries, context compaction, permissions, or workflow policy. Hidden subagents, visible side sessions, full trace timelines, exact per-call occurrence browsing, live status metrics, cross-session summaries, and billing/cost views remain outside the closed scope.

Validation

• pnpm run lint:web
• pnpm run type-check:web
• pnpm --dir src/web-ui run test:run - 76 files / 454 tests passed
• cargo check --workspace
• cargo test --workspace
• git diff --check
• git diff --cached --check

cargo check and cargo test still report the existing src/apps/desktop/src/tray.rs unused import warning, which is unrelated to this usage-report change.

Notes

The design doc still calls out one final UX signoff item: a real-App long-session smoke pass for scroll/jump feel. The branch keeps this explicit instead of overstating P2 as fully complete.