refactored graph for clear time based data display

[rssdev10]

Functional changes:

• Time-based X axis with labels. Points are placed by timestamp within an explicit time domain. Roll's domain is the window itself anchored to now, so a partly-filled window draws on the right instead of stretching to fill; Infinite spans the retained data. Axis labels added (-2m05s in Roll, HH:MM:SS in Infinite).
• Full span always drawn. The 600-sample cap is replaced with min/max decimation at ~2 points per pixel — no limit on how much time a plot may cover, and single-sample transients survive.
• Gaps render as gaps. A pause longer than 5 poll intervals breaks the polyline, so disconnects and paused logging are no longer drawn as straight lines through missing data.
• Mode switching is lossless. Roll↔Infinite is a view-only change and never touches the sample buffer; it is safe to toggle mid-run.
• Clear is the only action that hides data. Its epoch is now honoured in both modes (was Infinite-only, which made the two modes disagree), the button is shown in both, and the buffer is left intact so a CSV export after a Clear still contains everything. Clearing the samples buffer resets the epoch.
• Sample buffer holds a whole run. The fixed 7200-sample cap (~24 min) is replaced by a configurable retention, default 24 h, in Settings → Application. The graph and CSV export read the same buffer, so retention bounds both together. A buffer-span label shows how much history actually exists.