Tabbed details dialog: Turn, Session, History (0.5.0)

[charlesnutter]

Rebuilds the details dialog as tabs, laid out from the approved mockups. Ships as 0.5.0.

What changes

• Tabs: Turn, Session, History, switched with tab / shift+tab. /headsup session and /headsup history open those tabs; asking for the open tab closes the dialog, another tab switches to it.
• History tab replaces the history panel: ctrl+shift+h opens it. Fixed columns spread across the width, never wrapped; s switches this session / all sessions (all adds a model column); cost and cache columns only when some turn has them. The panel's code stays for one release, unused.
• Layout: xlarge, height fitted to content (up to 75% of the screen), a lighter panel inside the dialog's darker ring, a blank row under each section title, square bars, figures on shared columns (Tokens; a three-column engine grid). Colours are the mockup's on a dark theme, contrast-picked from the theme on a light one.
• Turn tab: time split bar and legend, a per-step timeline, steps with the engine's per-step rate, tokens, engine-only figures.
• Session tab: speed spread and trend, time split, tools by time (or why there are none), coverage, tokens, engine averages, sub-agents.
• Data consistency: tokens lead with the generated total everywhere; shares never round a real part to 0% or 100%.
• New module dialog.ts (pure): all three tabs as styled lines; the entry file only draws them.

Verified

• npm test (unit + e2e) and tsc --noEmit pass.
• Live on OpenCode 2.0.12 across all three tabs (screenshots in the conversation); every layout checked for overflow at 72, 80 and 84 cells against real history.
• Measured along the way: large is 88 cells; the dialog pads its own top by a row; a full-width line that overflows wraps onto a blank-looking row.

Not yet

• Tests for dialog.ts and the tabbed dialog, as a separate pass (per the testing rules in AGENTS.md).