feat(menu): reorganize sidebar with banded ordering and grouped headers

[antosubash]

Summary

Reorganizes the sidebar so installed modules cluster into coherent visual groups instead of an interleaved flat list.

• Banded MENU_ORDER — adopted stable ranges (10–99 content, 100–199 administration, 200+ system) and resolved two pre-existing order conflicts (settings vs. users at 30, datasets vs. file_storage at 40 — both moot post-merge but the framework-level convention now stands for any module that gets added back).
• group field on MenuItem — new optional group: str = "" ships through the existing get_for_user() payload to the frontend.
• Grouped sidebar rendering — SidebarLayout clusters consecutive items by group, prints an uppercase muted heading per non-empty group, and leaves ungrouped items (Dashboard) flat at the top. Each group's position is set by the lowest-order item that joins it, so backend-side reordering Just Works.
• Built-in module assignments
  • Content → File Storage (and would cover Products/Datasets if reintroduced)
  • Administration → Users, Feature Flags, Background Tasks
  • System → Settings
• Convention documented in docs/framework-conventions.md so future modules pick the right band/group.
• Scaffold updated — make new-module emits group="Content" by default.
• Two new tests in framework/core/tests/test_menu.py cover the default empty group and group serialization.

This branch was developed before upstream #96 removed the products and datasets modules; the merge accepted those deletions, so the order/group changes for those two modules are gone but the framework-level mechanism (and assignments for the surviving modules) is intact.

Test plan

• uv run pytest -q — 945 passed
• uv run python -m simple_module_core (sm doctor) — clean
• uv run ruff format --check . && uv run ruff check . && uv run ty check — all green
• make ci-js-typecheck — all workspaces green
• Manual visual check of the sidebar in make dev (skipped — no UI session in this run; reviewer to verify if desired)