Bug fixes and interface polish.

• Burn forecast: the "at this pace" estimate no longer disappears when Claude
  reports a fraction of a percentage point less than the previous sync, and it
  now follows the pace of the last ninety minutes so an early burst stops
  predicting a limit long after the conversation has gone idle.
• Token estimates: each sent message was counted twice, once from its structured
  content and once from a duplicate plain-text copy of the same content, which
  roughly doubled every estimate. Each message is now counted once.
• The meter no longer briefly disappears while a long prompt reflows the message
  composer, and two overlapping updates can no longer leave a second meter on
  the page.
• Each row of the detail panel is now coloured by its own percentage, so a
  weekly limit close to full is no longer shown in the calmer colour of the
  five-hour window.
• Consistent alignment, spacing, and keyboard focus styling across the detail
  panel in both the light and dark themes.

Added conversation token counting.

The meter now shows the current conversation token count and a daily token total. Token counts are calculated locally in the background script from Claude conversation data using the bundled tokenizer, then
stored locally in browser.storage.local.

No data is sent to any third-party service.