[Improve] Show Cost Analytics tokens as a trend line

[roomote-roomote]

​Created by Roomote. Follow up by mentioning @roomote-roomote, in the web UI, or in Slack.

What changed

• Replaced the grouped token bar stacks in Cost Analytics with one aggregate token trend line for each selected time bucket.
• Kept grouped cost bars and their existing drilldowns unchanged.
• Preserved per-group token counts and zero-cost token usage in the shared tooltip breakdown.
• Increased mobile axis space and used a clear continuous foreground-colored line so both USD and token scales remain readable without implying an error state.

Why this change was made

Showing cost and token breakdowns as adjacent stacked bars made the chart visually dense. A single token trend line makes total usage easier to follow while the tooltip retains the detailed relationship between groups, spend, and tokens.

Impact

Cost Analytics presents a clearer cost-versus-volume comparison without changing its table, filtering, grouping, token accounting, or cost drilldown behavior.

Screenshots

[roomote-community]

No code issues found. See task

_{Reviewed fdd3d96}

[roomote-roomote]

Updated the aggregate token line and points to use the theme foreground color in fdd3d96, preserving grouped cost and tooltip breakdown colors.