Stop the map's tiles showing a grid of dark lines

[ryanbarlow97]

What

Moving (and, more faintly, resting on) the map showed a grid of thin dark lines along the tile edges, on screens with a fractional device pixel ratio (Windows at 125 %/150 %, many laptops). At 100 % there were none.

Why

The tile images are clean. Chrome painted each <img> placed by left/top about 0.2 device px past its layout edge, so every tile's first column and row was ~20 % see-through and the page behind showed. Mid-drag the viewport translate was also fractional, so every tile edge straddled a pixel.

Fix

• TileLayer: tiles are placed with transform: translate(), which Chrome keeps exact.
• useMapViewport: a live pan (no scale on top of the resting zoom) sits on whole device pixels, as the resting position already did.

Overlapping tiles was rejected: prosperity tiles are ~40 % partial alpha, so overlap strips would darken.

Checked

Chromium against live map data (solid-black tiles on a white background, worst see-through at a tile edge):

	before	after
dpr 1.25, at rest	18 %	0 %
dpr 1.5, at rest	24 %	0 %
dpr 1.25, zoomed in, mid-drag	24 %	≤ 3 %
dpr 1	0 %	0 %

tsc clean; 1,427 map/viewport/admin/chronicle tests pass. Mid-pinch (scale ≠ 1) seams can still flash until the zoom settles.

🤖 Generated with Claude Code

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📥 Commits

Reviewing files that changed from the base of the PR and between fa996c1 and 51bde6e.

📒 Files selected for processing (2)

• frontend/app/components/map/TileLayer.tsx
• frontend/app/hooks/useMapViewport.ts

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