style(ui): restore rounding for buttons, comments, and company/project icons

Per feedback on PAP-186: containers should have hard edges but buttons,
comment containers, project icons, and company icons should keep rounding.

- Restore --radius-sm (6px) and --radius-md (8px) for buttons/inputs
- Keep --radius-lg and --radius-xl at 0 for cards/containers/dialogs
- Add rounded-sm to comment container divs in CommentThread
- Replace rounded-xl with rounded-[14px] on company icons (CompanyRail,
  CompanySettings) since --radius-xl is 0
- Fix brand color dot in Sidebar (rounded → rounded-sm)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ui/src/index.css

@@ -36,8 +36,8 @@
   --color-sidebar-accent-foreground: var(--sidebar-accent-foreground);
   --color-sidebar-border: var(--sidebar-border);
   --color-sidebar-ring: var(--sidebar-ring);
-  --radius-sm: 0px;
-  --radius-md: 0px;
+  --radius-sm: 0.375rem;
+  --radius-md: 0.5rem;
   --radius-lg: 0px;
   --radius-xl: 0px;
 }