fix(dotcom): hide section header when sidebar has a single section (tldraw#9416)

In order to avoid a redundant group header when the sidebar file list
only has one time-based group (e.g. only "Today", or only "Older"), this
PR hides the solo section's title via CSS.

Each section renders as a `sidebarFileSectionWrapper` element directly
inside the file list container, and empty groups render as `null` rather
than empty nodes. So "only one section is present" is exactly
`:only-child`, and a single CSS rule hides that section's header. As
soon as a second section appears, all headers return.

Closes tldraw#9412

### Change type

- [x] `improvement`

### Test plan

1. Open tldraw.com with an account whose files all fall into a single
time group (e.g. all created today).
2. Confirm the sidebar shows the file list with no "Today" header.
3. Create or open an older file so a second group appears.
4. Confirm both group headers ("Today", "Older", etc.) are now shown.

### Release notes

- Hide the sidebar file-group header when only one group is present.

### Code changes

| Section | LOC change |
| ------- | ---------- |
| Apps    | +6 / -0    |

Author: steveruizok

apps/dotcom/client/src/tla/components/TlaSidebar/sidebar.module.css

@@ -425,6 +425,12 @@
 	margin-bottom: 28px;
 }
 
+/* When a section is the only one rendered (e.g. just "Today" or just "Older"),
+   its header is redundant, so hide it. */
+.sidebarFileSectionWrapper:only-child .sidebarFileSectionTitle {
+	display: none;
+}
+
 .sidebarFileSection {
 	position: relative;
 	background: var(--tla-color-sidebar);