fix(canvas): make the channel sidebar feel instant

[raquelmsmith]

The channel sidebar felt slow in two ways. Both come down to the same backing query — ["canvas-channels"], populated by a paginated GET over the desktop file system — and this PR fixes each.

1. A newly-created channel was slow to appear

On create, the code only called invalidateQueries, forcing a full re-fetch to complete before the new channel showed up — even though the create API already returns the fully-formed channel.

Fix: createMutation.onSuccess now writes the returned channel straight into the cache, then invalidates to reconcile in the background. The sidebar selector already sorts folders alphabetically, so it lands in the right spot the instant the POST resolves — before navigation completes. A duplicate-id guard avoids a double insert if the 30s poll landed it first.

2. The channel list was slow to load on app start

getDesktopFileSystem() fetched every entry in the desktop file system — all dashboards and filed tasks too — page by page, up to 50 sequential round-trips, then filtered to type === "folder" on the client. On startup the sidebar waited for that whole tree walk.

Fix: filter to type=folder server-side and request a larger page (limit=200), so the common case is a single round-trip returning only channels. The endpoint already accepts type filtering (channel tasks use type=task). The returned set is identical to the prior client-side filter, so behavior is unchanged. Renamed getDesktopFileSystem → getDesktopFileSystemChannels to reflect the narrowed contract (its only caller is this sidebar).

Verification

• useChannels.test.tsx proves the created channel appears from the optimistic write while the refetch is hung.
• pnpm --filter @posthog/api-client typecheck, pnpm --filter @posthog/ui typecheck, the UI test, and Biome all clean.

🤖 Generated with Claude Code

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 012dc62.}

[greptile-apps]

Prompt To Fix All With AI

Fix the following 2 code review issues. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 2
packages/ui/src/features/canvas/hooks/useChannels.test.tsx:17-20
**Mock object cast avoids required interface fields**

`{ id, path, type: "folder" } as unknown as Schemas.FileSystem` sidesteps any additional required fields in the `FileSystem` interface. If the interface gains a required field that the production code relies on, these tests will silently pass with incomplete mocks. Prefer populating all required fields so TypeScript catches drift (or use a typed factory that provides safe defaults).

### Issue 2 of 2
packages/ui/src/features/canvas/hooks/useChannels.test.tsx:37-63
**Duplicate-ID guard has no test**

The PR description explicitly calls out "a duplicate-id guard avoids a double insert if the 30s poll landed it first," but only the happy-path insertion is exercised here. A second test case — seed the list with the created channel already present, call `createChannel`, and assert the list length hasn't grown — would give that guard the same coverage as the optimistic-insert path.

_{Reviews (1): Last reviewed commit: "fix(canvas): show newly created channel ..." | Re-trigger Greptile}

[greptile-apps]

_{Reviews (2): Last reviewed commit: "test(canvas): type the channel fixture a..." | Re-trigger Greptile}