Make dark-theme canvas rendering match light where the drag artifact lives

[dovvnloading]

Problem

Dragging a node slowly makes its connection line visibly step out of sync with the node - but only in the dark theme. A full audit of every difference between the two themes (design tokens, the entire stylesheet, all application code, the canvas library's internals, the desktop shell, and external reports of the same symptom) established two things. First, no code executes differently by theme: the lag mechanism - the node moving on the graphics processor's fast path while the connection line is redrawn each frame - exists identically in both themes. Second, exactly three rendering differences make dark display that lag while light hides it:

1. The connection line in dark was a bright stroke on a near-black canvas at roughly 2.07:1 contrast, versus light's soft 1.68:1 gray-on-gray. At a width of 1.5 pixels the line also falls into the renderer's thin-line smoothing regime below about 67 percent zoom, whose stepping is measurably harsher for a light line on a dark background than the reverse. Same misstep, dramatically different visibility.
2. Dark's card shadows were 2.2 times denser than light's - the only structural asymmetry in the whole theme palette - giving the dragged card a much crisper silhouette to judge the line against.
3. The page root had no background of its own, so anything shown before content paints resolved to the window's backdrop rather than the theme's canvas color.

The audit also found that the composition change in #323 never took effect: the window library passes its own feature-disabling switch, duplicate switches resolve last-one-wins, and the flag was silently discarded.

Change

• New per-theme connection stroke token: dark gets #444444, which reproduces light's contrast ratio on the dark canvas; light keeps its current appearance. Stroke width goes from 1.5 to 2 to stay out of the thin-line smoothing regime at working zoom levels, with high-quality smoothing pinned explicitly.
• Dark's shadow values now equal light's, removing the palette's only structural asymmetry.
• The page root paints the theme's window color opaquely in both themes.
• The launcher now merges both feature names into a single switch so the composition setting actually reaches the browser regardless of which argument list wins.

Test plan

• Full frontend check passes: schema, types, lint (0 errors), 1927/1927 tests including the token parity and contrast guards, production build, bundle size
• Launcher passes a syntax check
• On the affected machine, dark theme: relaunch the desktop app and drag a connected node slowly - the reporter's reproduction, requiring visual confirmation