feat(pm): multi-project PMs (1:N) with project switching, scoping validation and org multi-select

Task 1 — PM 1:N + access-loss handling:
- projects.pm_user_id had no UNIQUE constraint (verified live + in schema); added a lookup index via init_db
- auth: UserContext.project_ids = every project of a PM (oldest first); project_id stays the first for compatibility
- GET /users/me/projects — assigned projects of the caller (drives the switcher + revocation validation)
- login / /users/me / /users list now return the full projects array per user
- /inventory/* and /pm/* scopes accept an explicit ?project_id=… and verify the PM manages it (403 otherwise); without the param they fall back to the first project; admin/manager use it as an optional filter
- POST /users/{id}/project is multi-aware: assigns one project (replacing that project's previous PM) and KEEPS the user's other assignments; null clears all
- PATCH /projects/{id} accepts pm_user_id (admin / procurement manager only)
- frontend: auth store holds projects + the active project (persisted via the existing localStorage store); RequirePmProjects re-validates on load, falls back when the active project was revoked, and shows a dedicated no-projects screen with logout (no redirect loop, no re-auth)
- Panels menu: «مدیریت انبار» gets a ChevronDown accordion listing the PM's projects with «✓ فعلی»; selecting one switches only the active project and reloads the section data
- inventory/pm API calls now send the active project_id explicitly

Task 2 — Organization page:
- new searchable ProjectMultiSelect combobox (chips with ×, live Persian filtering via normalize, assigned projects hidden) replaces the single select for PM users
- add = PATCH project pm_user_id=<user>; × = pm_user_id=null; table and project cards sync immediately; RBAC unchanged (role column admin-only, combobox editable for admin + procurement manager)

Verified locally: 17 scripted checks — multi-project assignment without unique conflicts, /users/me/projects, explicit project_id authorization (foreign project 403 on /inventory, /pm/on-the-way, /pm/receipts), PM blocked from pm_user_id changes, legacy assign endpoint keeps other assignments, revocation removes the project from the list, zero-project PM gets an empty list and a guarded inventory endpoint. All test data cleaned up afterwards.

backend/app/schemas/procurement.py

@@ -52,6 +52,9 @@ class ProjectUpdate(APIModel):
     name: str | None = Field(default=None, min_length=1)
     description: str | None = None
     start_date: date | None = None
+    # PM assignment (1:N — one PM per project, a PM may own many projects).
+    # Admin / procurement manager only; None unassigns this project's PM.
+    pm_user_id: UUID | None = None
 
 
 # ---------------------------------------------------------------------------
@@ -210,7 +213,11 @@ class UserRoleUpdate(APIModel):
 
 
 class UserProjectUpdate(APIModel):
-    """Assign/unassign the user's single project (project_id=None unassigns)."""
+    """Assign the user to one project (multi-project aware).
+
+    project_id given → that project gets this PM (previous PM replaced,
+    the user's other projects kept); None → clears every assignment.
+    """
 
     project_id: UUID | None = None