feat(timeline): day-resolution tooltip + phone-friendly rail and taps

[bndct-devops]

Overnight run PR 2/12 — stacked on #132; merge in order.

What

• Day-resolution tooltip: hovering a bar now resolves the exact day under the cursor — "15 May · 22m", or "no reading" on a gap day — as an accent line under the book totals. You ran into this gap while testing: the ticks encoded day data but the tooltip only answered at book level.
• Mobile pass: below 480px the series rail narrows 200→96px so the ribbon keeps most of the screen; on touch devices the first tap on a bar shows the tooltip and a second tap on the same bar opens the book (tap elsewhere or scroll dismisses). Previously any tap navigated instantly, making "inspect" impossible.

Dropped from scope, deliberately

The book-page activity sparkline from the idea sheet is not in here: while building it I found the per-book stats hero already charts per-day activity for both data sources — compute_book_reading_stats synthesizes sessions from imported page-stats, so even device-only books get the hero with its day chart. The strip would have been a duplicate chart directly under an identical one. Marked accordingly on the sheet; no code shipped for it.

Verification

• WebKit iPhone 13 (touch): first tap keeps you on /stats with the tooltip up (screenshot shows the new day line), second tap navigates to the book. Narrow rail active.
• Chromium desktop: hover tooltip shows "May 15 · 22m" day line.
• Frontend build/typecheck clean. No backend changes.