Ideas board: vim navigation + visual-mode region select (M3)

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Keyboard-native navigation and region selection for the ideas board (depends on #156 the engine endpoint, and #157 the board scaffold). This is what turns "analyze the whole 1Y window" into "analyze a candle range I selected".

Model (vim)

Board level — h/j/k/l move the selection between elements (nodes); Enter (or zoom) focuses a chart node, switching into chart mode. Esc returns to board level.

Chart level (a focused chart) — a candle cursor:

• h / l = move ±1 candlestick (left / right)
• H / L = ±5 candlesticks
• {count}{motion} = ±count, e.g. 10l = 10 right, 10h = 10 left (standard vim count motions)

Visual mode — v anchors at the cursor; h/l (and counts) extend the selection over a candle range, rendered as the "analysed window" (in-range candles bright + blue/orange rails, the rest dimmed to context). Enter / :analyze fires GET /api/backtest/optimal-entry/{symbol}?from=&to= for the selected window and pins a way-to-trade card wired to the region. Esc cancels.

Notes / approach

• Reuse the unified region vim-nav philosophy already in vim-nav.js; here it runs inside the React Flow board (board level) + inside a focused chart (candle level).
• Use lightweight-charts coordinate APIs (logical range / timeToCoordinate) to place the cursor + the range overlay; the selected range maps to the endpoint's from/to.
• The selection is keyboard-native marquee — same result as a mouse drag, but vim-first (mouse = failure).

Acceptance

• Cursor moves by 1 / 5 / count; v selects a range with the analysed-window rendering; Enter analyses exactly that window and pins the card; Esc exits cleanly; no element is unreachable by keyboard.