feat(context): draw the usage meter as a full ring

[luoxuanzao]

Summary

• The context usage meter beside the message input swept a 240° arc, leaving a
  gap at the top: the shape read as an unfinished circle, and the same reading
  covered less visual ground than a ring would. The gauge is now a complete
  circle that fills clockwise from 12 o'clock, drawn with two <circle>
  elements instead of an arc path.
• Only the shape changes. The percentage label, the hover tooltip with the
  token counts, and the pale-red warning state above 80% behave as before, and
  the meter is still not clickable — no panel is added here.
• Documented under [Unreleased] in CHANGELOG.md (Changed).

Verification

• npm run typecheck
• npm run lint
• npm run test (171 suites, 3600 tests)
• npm run build
• npm run release:check
• npm run audit:prod
• Tested affected qodercli behavior against a real CLI when applicable —
  no CLI behavior is involved in this change; the ring was checked in Obsidian
  against the built main.js (empty at 0%, filling clockwise from the top,
  warning tone above 80%).

Safety

• No credentials, private vault content, internal URLs, or personal paths are included
• User-visible changes are documented in CHANGELOG.md