Render at the terminal's real cell pixel size (DPI-aware), with fonts sized to match #18

How rendering is sized now:

• The canvas is cols*16 × rows*32 px. That's about 1× on a Retina Mac (a cell is about 16×34 device px), so the terminal stretches the image a little.
• The code comment claims "2x supersample", which is wrong.
• MAX_PIXELS = 2M (only needed for chafa's WASM) pushes large iTerm2 charts below 1×. For example 200×50 cells becomes 2000×1000 px, which is 10×20 px per cell.
• renderer.ts sets fontPx = max(10, height/40), so axis labels are about ⅓ of a cell tall, while terminal text is about 0.7 of a cell. Line widths are tripled as a separate workaround.

Proposed fix:

1. Get the cell size in pixels with CSI 16 t (reply CSI 6;h;w t, supported by kitty, iTerm2, WezTerm, foot and ghostty; check xterm.js). Read the reply before Ink attaches its stdin parser, or it leaks into the app's input like the kitty replies did (Fix kitty reply leak, inline-image cleanup, cursor handling and kitty image IDs #15). Alternatively accept a cellPixelSize/devicePixelRatio prop, with a sensible default of 2 on macOS.
2. Render at exactly cols*cellW × rows*cellH device pixels so the terminal doesn't resample.
3. Let uPlot do the scaling. Set the DOM shim's devicePixelRatio before the first import('uplot'): uPlot reads pxRatio once at import, and the shim never fires dppxchange. Then pass CSS-pixel width and height. That replaces the height/40 and width*3 workarounds.
4. Drop MAX_PIXELS for the PNG path and keep a sane MAX_DIM.