feat(portal): route canvas header nav

[07prajwal2000]

What

• Canvas header now carries navigation instead of dev-era metadata:
  • back link to the project's route list
  • new RouteSwitcher — pagination-style stepper (prev / next) showing method + path of the open route, with a dropdown to jump straight to any sibling route and a n / total counter
  • dropped the read-only toggle and the blocks/edges/unsaved counters; the Save button's disabled state already communicates pending changes
• CanvasWorkbench gains a headerLeft slot so the custom-block canvas keeps its plain title.
• MethodSwitch moved off hardcoded hex onto theme tokens — renders correctly in light and dark.

Notes

• The switcher requests perPage: 50 (the server's max); when more routes exist the counter reads 50+ and the routes list screen is the way to reach the rest. The open route is fetched by id separately so it stays selectable even if it sits past page 1.
• Skipped: search/filter inside the dropdown — add it when projects routinely exceed one page.

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