Match chat sessions to sites by site id instead of path

[bcotrim]

Related issues

• Related to STU-1985 (one slice of the sidebar-redesign split — matching semantics only)

How AI was used in this PR

Claude implemented the change end-to-end from a scoped brief: traced the placement/matching flow, made the code changes, and extended the unit tests. I reviewed the diff via a guided code tour and steered the follow-up (extending id-matching to all call sites through a shared helper).

Proposed Changes

Chat sessions are bound to local sites via placements in ~/.studio/app.json, which have always stored a validated siteId — but the actual matching (sidebar grouping, empty-draft reuse, session header/preview owner lookup, and the main process' owner-server resolution) compared filesystem paths. Since site paths are name-derived and get reused, deleting a site and creating a new one at the same folder made the new site silently inherit the dead site's chat history.

This PR switches matching to the site id, consistent with everything else per-site in app.json (e.g. siteMetadata). All matching now routes through one shared, dependency-free helper (packages/common/ai/sessions/owner-site.ts) used by the renderer, the router, and the main process:

• A recreated site at the same path starts clean; chats from the deleted site show under Unassigned instead of leaking into the new site.
• "New chat" no longer reuses an empty draft that belonged to a different (deleted) site at the same path.
• Session header, dashboard preview, initial-route redirect, and environment switching resolve the owner site by id too, so a stale path can never rebind a session to the wrong site.
• A session whose owner site no longer exists is orphaned visibly (Unassigned / plain site-name header), never adopted by another site and never dropped.
• ownerSitePath/ownerSiteName are kept for display, and path matching remains as a defensive fallback for placements written before siteId existed (placements are read from user-editable JSON without schema validation), so no existing chats are dropped.

Testing Instructions

Unit tests cover the matching semantics:

• npm test -- packages/common/ai/sessions/tests apps/ui/src/components/site-list

Manual (agentic UI, apps/ui):

1. Create a site and start a chat in it — it groups under that site in the sidebar.
2. Delete the site, then create a new one with the same name (same folder path).
3. The new site should show "No active chats"; the old chat should appear under Unassigned — previously it leaked into the new site.
4. Open the old chat from Unassigned — the header shows the stored site name as plain text (no site dropdown), and prompting it does not target the new site.
5. Draft reuse: open "New chat" for a site twice without prompting — the empty draft is reused. Recreate a site at the same path — the new site gets a fresh draft, not the old site's.
6. Legacy fallback: remove siteId from one entry in aiSessionPlacements in ~/.studio/app.json — that chat should still group under its site via the path.

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

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