Show synced context usage rings across harnesses

[wingleeio]

Adds a context ring beneath the composer for every selected conversation, including remote and project-less chats. Hover shows measured tokens and remaining capacity; the ring turns amber at 75% and red at 90%. Unknown usage is distinct from measured zero, and an open hover card updates live.

The host normalizes context separately from billing totals and persists one atomic snapshot in the chat document. Existing document sync and transcript watches carry it to other devices, including context-only updates, reconnects, and document rebuilds. Rendering needs no polling or local CLI access.

Claude Code, Codex, OpenCode, and ACP harnesses supply measurements when their protocols report them. Cursor's pinned SDK only exposes billed per-turn totals, so it shows unavailable instead of a guessed percentage.

Validation:

• 89 document, 130 engine, and 617 UI unit tests passed.
• Harness unit and integration suites passed; credential-dependent tests remain ignored.
• Added an engine integration test proving post-turn compaction updates context without reopening the turn or creating transcript rows.
• Two isolated engines through a local relay: identical 64,000/200,000 and 184,000/200,000 snapshots, post-turn compaction to zero, and viewer restart with the host stopped.
• Native app build and git diff --check passed.

Screenshots are from the native desktop connected to the viewing engine. A separate host replayed deterministic Claude protocol fixtures through the local relay; the displayed measurements are test data.

Measured usage, dark mode:

Near capacity, light mode:

Unavailable usage:

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