feat(nav): two-level sidebar on Build/Operate taxonomy — Know group, Users under Settings, rail flyouts

[esafwan]

Summary

Reworks the sidebar on top of develop's Build / Operate taxonomy, taking the interaction idea from #283 (accordion + collapsed-rail flyouts) and applying it to the current UI structure instead of the old flat nav it was built on. Supersedes / closes the need for #283 (which was based on the pre-taxonomy UI).

New structure

Group	Design	Items
Dashboard	flat item	—
Build	label group (unchanged)	Agents, Agent Prompts, Flows
Know	label group (new)	Tables (/data), Sources (/knowledge)
Operate	label group (unchanged)	Chat, Executions, Artifacts
Settings	collapsible group	AI Providers, Models, Agent Summary Prompts, Console, Integrations, Integration Services, MCP Servers, Roles, Users

Key taxonomy decisions:

• "Know" label — keeps the terse verb register of Build/Operate (BUILD, OPERATE, KNOW).
• "Documents" renamed to "Sources" — the page is any vector/FTS-backed retrieval store (Chroma, sqlite-vec, pgvector), not literal documents, and the name scales to future source types (Files, Repos, Drives) nesting under Know as additional ingestion surfaces feeding the same retrieval layer.
• Users moved from People into Settings — user management is an admin task, not a daily destination; the single-item People group is gone.
• Know stays a flat label group (only 2 items — a collapsible would cost a click for no benefit); Settings is the only collapsible group, where collapsing earns its keep at 9 items.

Mechanics

• New NavCollapsibleGroup component (frontend/src/components/nav-collapsible.tsx): renders as an accordion section when the sidebar is expanded, and as a DropdownMenu flyout popover when the rail is collapsed — sub-items stay reachable without expanding the rail (replaces the old "click Settings to force-expand the rail" workaround).
• The active route auto-opens its group; per-item capability gating is preserved (users.manage, agent.view_all, etc.).
• app-sidebar.tsx is now just group definitions + capability filtering — future taxonomy changes are one-line edits.

Testing

• Built and live-verified on a disposable bench provisioned as a full data restore of a production-like site (47 agents, real conversations): all routes resolve, no dead links, flyout works on the collapsed rail, chat history loads clean.
• Screenshot review of expanded sidebar, Settings accordion, and collapsed-rail flyout — all render as designed.

Screenshots

Expanded sidebar — Build / Know / Operate label groups, Settings collapsible:

Collapsed rail — Settings flyout popover (all 9 items incl. Users):

Notes

• Route paths are unchanged (/knowledge, /data, /users, ...) — only sidebar lingo and grouping moved, so no link breakage.
• The /knowledge page itself still titles "Knowledge Sources"; renaming the page title to match "Sources" can follow separately if the lingo sticks.

[esafwan]

Superseded by consolidated Apps/workspace/navigation PR #471. Navigation taxonomy and collapsed-rail behavior are preserved there alongside the Apps launcher integration.