R8a: unify canvas chrome spacing, give the minimap real colors, hide the token counter by default

[dovvnloading]

Problem

Three related issues with the floating chrome around the canvas:

1. Pins/search/view-popover/plugin-picker were anchored at an inconsistent 8px edge inset, the token counter/notification used 16px, and the minimap used React Flow's own baked-in 15px Panel margin - three near-identical values with no stated relationship, none of them tied to this app's own design system.
2. The minimap rendered every node as React Flow's own default plain rectangle - flat, undifferentiated boxes with no relationship to the actual canvas content ("just white boxes").
3. The token counter overlay defaulted to visible, with no easy way to discover it's actually a real, toggleable setting.

Change

• Spacing: added one canonical --gl-chrome-inset (16px) token, the same pattern already used for the z-index scale. Every corner/edge-anchored chrome element (pins, search, view popover, plugin picker, token counter, notification, minimap) now uses it. Also fixed a stale comment on the z-index scale (claimed pins used the canvas-chrome tier; the real code has used the app-layer tier for a while) and removed .scene-pins, a fully dead CSS block from a pre-Popover-refactor pins implementation with zero remaining JSX references.
• Minimap: now passes real nodeColor/nodeStrokeColor to React Flow's <MiniMap>. note/frame/container nodes show their actual assigned color (the same palette the color picker writes); every other kind gets one deliberate neutral tone instead of the flat default; the currently selected node is highlighted. Colors are read from the real design tokens at render time, not hardcoded hex.
• Token counter default: now off by default (fresh installs and any settings file missing the key). The toggle itself was already fully real and working (persisted setting, wired Settings checkbox) - this only flips which state a new user starts in. Existing users who already have it explicitly enabled are unaffected.

Test plan

• Backend: 995 passed, including updated coverage for the new default and a real state-transition test for the setter
• Frontend: tsc --noEmit clean, lint 0 errors, 794 tests passed, build succeeds
• Verified live against the real, running app: all chrome layers report a consistent 16px inset via computed styles; the minimap's real rendered SVG nodes show the new neutral fill/stroke colors (confirmed on a loaded chat with 13 real nodes); confirmed via an isolated script that both a fresh install and a pre-existing settings file missing the key now default show_token_counter to False, while the real user's already-explicit true value in their existing session.dat is correctly left untouched