Heptabase

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Build me the visual thinking canvas that replaces Heptabase.

The premise is that **arrangement carries meaning** — two cards side by side say something a list cannot. That is real, and the difficulty is entirely in the canvas being fast and reliable.

STACK
- Node 20+ with Fastify for the server half
- SQLite through better-sqlite3, WAL mode, FTS5 across cards
- Canvas or SVG in the browser with vanilla JS; no framework
- Caddy in front

THE DATA MODEL
- cards: id, title, body_md, created_at, updated_at
- boards: id, name, viewport_json
- placements: id, board_id, card_id, x, y, w, h, z — **the same card can be on several boards in different places**, which is the point
- connections: id, board_id, from_card_id, to_card_id, label, kind
- groups: id, board_id, title, bounds — a named region
- Cards exist independently of boards. **A card removed from a board is not deleted**, which is the difference between thinking space and a drawing

THE CANVAS, WHERE ALL THE WORK IS
- Pan and zoom that feel right: momentum, zoom to cursor, and a fit-to-content key
- **Render only what is visible, and simplify below a zoom threshold** — a card zoomed out is a coloured rectangle with a title, not a rendered document. Without this it dies at two hundred cards
- Snap to a grid and to other cards' edges, with alignment guides
- Multi-select by rubber band, move and align as a group
- Every operation undoable, including a group move. **Undo is not optional on a canvas** — a mis-drag moves forty cards and without undo that is unrecoverable

CARDS
- Markdown, edited in place, with links between cards autocompleting
- A card shows which boards it appears on, so nothing is lost by being moved off one
- Backlinks with the surrounding sentence

THE FEATURES THAT MAKE IT MORE THAN A WHITEBOARD
- Full-text search across every card, with results highlighted on the current board and a jump to any other
- **A list view of every card, so the canvas is never the only way to find something.** A purely spatial system loses things permanently, and this is the guard against that
- Connections with labels, because an unlabelled arrow means nothing a month later

PERFORMANCE RULES
- Positions written on drag end, not during. Batch them
- Sparse coordinates, indexed by board, so loading a board is one query
- Card bodies loaded lazily; only titles are needed to draw

SAFETY
- Export every card as markdown with a JSON file describing the layout. **The writing must be recoverable without the canvas**
- Append-only change log, so a bad drag or a deletion is reversible

WHAT MATTERS MOST
Windowed rendering with zoom simplification, real undo, and a list view alongside the canvas. The first two make it usable; the third stops it losing your work.