feat(design): a project reports drift in its own recorded components

The check has taken a project id since it was written — check_snippets_against_
system(user_id, design_system_id, project_id=0), and the route has always read
?project_id=. Nothing on the frontend ever passed one and no project-side
surface existed, so the capability shipped and stayed unreachable.

A Design tab on the project, beside Systems and Rules, reporting three things
per snippet:

  no such token     var(--x) the system doesn't declare. Renders as NOTHING —
                    no error, no failing test, just an element quietly unstyled.
                    Leads for that reason.
  defines its own   a component minting a custom property instead of reaching
                    for the shared one. This is the DRY finding and the reason
                    the surface exists: the codebase re-solving a solved
                    problem, one component at a time, visible only when someone
                    changes the shared value and half the components don't move.
  write the token   a literal the sheet says to stop writing, paired with what
                    to write instead.

Three empty states, kept distinct, because collapsing them is how a check comes
to sit dead: no design system bound, no snippets recorded (nothing was
checked), and checked-and-clean. The last one says how many were checked.

Bound to the SAVED pointer rather than the sidebar picker's draft, so an
unsaved change can't make the tab report against a system the project isn't
using.

Scope is recorded code, per the operator: snippets are what Scribe holds, and a
repository's own sources are checked where they live, by that project's CI.

Step 3 of milestone #274.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UaYUaouG9jjhATyuxCKrQs

frontend/src/api/designSystems.ts

@@ -200,6 +200,14 @@ export interface SnippetCheck {
   findings: SnippetFinding[];
 }
 
-/** Which recorded snippets disagree with this design system's sheet. */
-export const checkSnippets = (id: number) =>
-  apiGet<SnippetCheck>(`/api/design-systems/${id}/snippet-check`);
+/** Which recorded snippets disagree with this design system's sheet.
+ *
+ *  `projectId` narrows to the snippets one project owns — which is how a
+ *  project asks about its OWN code. Omit it to check every project, which is
+ *  the right default from the system's side: a component recorded elsewhere
+ *  still has to use the same tags. */
+export const checkSnippets = (id: number, projectId?: number) =>
+  apiGet<SnippetCheck>(
+    `/api/design-systems/${id}/snippet-check`
+    + (projectId ? `?project_id=${projectId}` : ""),
+  );