feat(tafsir): step between verses and resize the floating card

[ask-786]

Summary

Previous/next verse — both the floating card and the side panel get chevron buttons in their header.

• Steps one verse at a time and stays within the current Surah (disabled at its first/last verse).
• One verse at a time even where an edition comments on a run of verses, so the recitation strip plays every verse. The label now reads e.g. 2:3 on 2:1–5, so a step inside a run is still visible.
• The reader scrolls to the new verse only when it is outside the visible area (revealAyah in follow-scroll.ts, sharing the lookup with recitation follow).
• The floating card stays where it is while stepping, so the buttons stay under the pointer; focus stays on the pressed button.

Resizable floating card

• Bottom-right grip: drag, arrow keys (Shift for bigger steps), double-click to reset to automatic.
• The size is saved for every card: new settings tafsir_card_width / tafsir_card_height (0 = automatic) in the TS defaults, Rust model, load_settings and schema.sql defaults. No migration needed.
• Never extends off screen: growth stops at the window edge while dragging, and the saved size is refitted to the window on every placement and window resize.

The side panel was already resizable (left-edge handle) and is unchanged.

Test plan

• Step through a Surah in both surfaces. Buttons disable at its first and last verse.
• Step through a grouped-commentary run. Audio follows each verse, and the label updates.
• Step to an off-screen verse, in list and Mushaf view. The reader scrolls; a visible verse doesn't move the page.
• Resize the card, close and reopen, restart the app. The size persists.
• Shrink the window with a large saved card size. The card stays fully on screen.
• Double-click the grip. The card returns to the automatic size.

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