Make the default node font color follow the theme

[dovvnloading]

Problem

The canvas node font color is scene state (font_color, applied as --gl-node-font-color on the canvas wrapper by useCanvasFontVars). Its default was the literal "#F0F0F0" - a white chosen for the dark theme - so under the light palette every chat, thinking and markdown body rendered near-white text on white cards. A stored hex cannot adapt to a theme.

Reproduce on main: load any graph with the theme set to Match System and emulate prefers-color-scheme: light.

Change

The default is now the empty string, meaning "follow the theme". An explicitly chosen color still stores its hex and applies as-is in both themes.

• backend/domain/graph.py: font_color defaults to ""; setFontColor("") round-trips as the reset-to-theme value. font_color is in-memory session state with no persistence path, so there is no migration.
• useCanvasFontVars.ts: "" removes --gl-node-font-color rather than writing an empty value, so the existing CSS fallback - var(--gl-node-font-color, var(--gl-surface-text-primary)) - resolves to the active palette's body-text token, which the token contrast test already holds to WCAG AA in both themes. Writing "" would suppress the fallback: an empty custom property is still a set property.
• ViewPopover: the font-color row gains a leading "follow the theme" swatch, active when unset; the readout shows Auto instead of an empty hex; the font preview falls back to the theme token; Reset to Defaults returns to "" rather than to the dark-theme white; and the native color input is fed a valid hex when the stored value is empty (a color input coerces "" to #000000 with a console warning).

Test plan

• test_canvas.py: new round-trip - default is "", an explicit hex stores, "" resets. 326 backend canvas/migration tests pass.
• useCanvasFontVars.test.tsx, new: an explicit color is written as-is; "" removes the property (asserted as absence, since an empty value would reintroduce the bug in a new shape); family and size still apply when the color is theme-following.
• ViewPopover.test.tsx: the Auto swatch renders and reads active when unset; picking it commits "" through the debounce; an explicit color shows its hex with Auto unpressed; Reset commits "". Writing the Reset test exposed that the store double never mocked clearFilters - Reset threw an unhandled TypeError in jsdom - fixed in the double.
• npm run check against a clean checkout of this branch: 2135 pass. pytest for the touched backend suites and ruff clean.
• Verified live against the built bundle in both themes: dark renders #E0E0E0 node text with no --gl-node-font-color set; flipping to light renders #1F1F1F on light cards; the View panel shows the active Auto swatch and Auto readout.