Add toggleable per-message token usage badges in chat UI

[Copilot]

This updates the context estimator to show per-message token usage directly in the conversation view (enabled by default). Each badge displays estimated tokens and that message’s share of the selected model context window.

• Overlay controls

  • Added a persisted Per-message usage toggle in the Overrides section.
  • Introduced showPerMessageUsage in settings defaults/storage handling.
  • Set per-message usage display to enabled by default.

• Estimation pipeline

  • Extended estimate output to include per-message { tokens, contextPercent } when enabled.
  • Reused existing estimator selection/fallback behavior (Precise/Fast).
  • Added lightweight memoization for repeated message text within a calculation pass.

• Message-level UI rendering

  • Rendered compact badges next to message nodes in the format x tokens, x%.
  • Implemented idempotent badge updates/removal to avoid DOM-observer churn loops.
  • Added scoped styles for badge placement/appearance and checkbox layout.

• Docs

  • Updated README usage/features to mention per-message usage display.
  • Added a session devlog entry and index link.

// per-message usage shape produced by estimate engine
[
  { tokens: 26, contextPercent: 0.08125 },
  { tokens: 11, contextPercent: 0.034375 }
]

// rendered badge text
"26 tokens, 0.08%"

[namerror]

The displayed token label should be "x tokens, x%".

Also, enable display of per message token by default

[Copilot]

The displayed token label should be "x tokens, x%".

Also, enable display of per message token by default

Updated in commit 68c3232: badge text now renders as x tokens, x%, and per-message usage is enabled by default. Screenshot: https://github.com/user-attachments/assets/dca859cc-c39d-4bad-8cce-a58d90db1676