perf(desktop): restore project context during startup (#6939)

## Summary
Project channels previously appeared as ordinary channels until the
community-wide project enumeration completed, delaying the contextual
right rail most noticeably on larger relays.

- Restore the last fully validated, relay-and-identity-scoped project
collection immediately while keeping live relay data authoritative.
- Resolve the active channel's project home through scoped
`#buzz-channel` queries instead of waiting for the complete project
scan.
- Keep snapshots aligned with community removal and relay reconnect
invalidation.

### Related issue
None found.

### Testing
- Pre-push `file-size-check`, `desktop-check`, `desktop-typecheck`, and
`desktop-test`
- Project enumeration, snapshot persistence, and relay invalidation unit
coverage
- Targeted Playwright scenario: project sidebar rows open the home
channel and nest extra channels

No numeric startup benchmark was captured; this draft validates the
cache and scoped-query behavior while leaving timing measurement for
review.

---------

Signed-off-by: Thomas Petersen <thomasp@squareup.com>

desktop/src/features/projects/useAddProjectRepository.ts

@@ -18,6 +18,7 @@ import {
   addRepositoryToProject,
   eventToRepository,
 } from "@/features/projects/projectModels";
+import { markProjectDataAuthoritative } from "@/features/projects/projectSnapshot";
 import { publishProjectOwnerAnnouncement } from "@/shared/api/projectGit";
 import { relayClient } from "@/shared/api/relayClient";
 import type { RelayEvent } from "@/shared/api/types";
@@ -315,6 +316,7 @@ export function useAddProjectRepositoryMutation() {
     mutationFn: (input: AddProjectRepositoryInput) =>
       addProjectRepository(input),
     onSuccess: ({ previousProjectId, project }) => {
+      markProjectDataAuthoritative(project, "local-write");
       if (previousProjectId !== project.id) {
         queryClient.removeQueries({
           exact: true,