R8a: gate notifications on type preference, wire up node font styling (findings #10, #11)

[dovvnloading]

Problem

Two settings from the R8a audit persisted real state that nothing consumed:

• Finding Modernize cloud and Ollama model settings #10: Settings' Info/Success/Warning/Error notification-type checkboxes wrote real preferences via SettingsManager.set_notification_preferences, and get_notification_type_enabled already existed to read them back, but NotificationState.show() set visible = True unconditionally. Unchecking a type had no effect on whether its banner appeared.
• Finding Modernize Graphlink Composer to a production-grade request surface #11: The View popover's FONT section (family select, size slider, color swatches) already round-tripped real setFontFamily/setFontSize/setFontColor intents into scene state, but nothing on the frontend ever consumed scene.fontFamily/fontSizePt/fontColor as CSS. Changing any of the three controls visibly changed nothing on the canvas.

Change

• backend/notifications.py: register_notifications now takes an optional SettingsManager. NotificationState.show() early-returns (leaving any currently-visible banner untouched) when the message's type is disabled in preferences. The gate lives in the one method every existing notifications.show(...) call site already goes through (canvas.py, agents.py, chat_library.py, settings.py, plugins.py, autosave.py, crash_recovery.py), so all of them are covered without individual changes. backend/app.py now passes the session's real settings manager through.
• web_ui/src/app/canvas/SceneCanvas.tsx: writes scene.fontFamily/fontSizePt/fontColor onto the canvas wrapper as --gl-node-font-family/-size/-color custom properties. fontSizePt is points (backend field font_size_pt, range 8-16, default 9) — pt is a real CSS unit, so no conversion needed beyond the correct suffix.
• web_ui/src/app/styles.css: .scene-node, .scene-node-title, and .scene-node-body reference the new tokens with their prior literal values as fallbacks. Scoped to the shared base rules (title bar, generic body text) rather than each node kind's own content styling, so markdown heading hierarchy and monospace code blocks keep their own intentional typography.

Test plan

Layer	Result
npx tsc --noEmit	clean
npx vitest run (full suite)	868/868 passed
python -m pytest -q (full suite)	1041/1041 passed (8 new)
npm run lint	clean (0 errors)

New coverage: backend/tests/test_notifications.py (gating on/off per type, a suppressed show() doesn't clobber a currently-visible banner of a different type, dismiss() unaffected by preferences, register_notifications wiring, and the pre-existing no-settings-manager call sites still always show).

Live-verified end to end against a running instance (real WS round-trip, not a mock): dragging the font-size slider, picking a font family, and picking a font color each immediately restyled a real canvas node's title and body text to the new values; the initial page load already reflected the backend's default font settings correctly.