Auto-assignment improvements: fixes "General trap", stale topic on edit, and misleading placeholder copy

New Memories captured through the web app are now correctly auto-assigned to a matching Topic in your Personal Space — the previous web capture path was querying a stale location and assigning nothing (MCP captures via Claude were unaffected). The auto-assignment decision rule is now stricter: a Memory only moves to a Topic when it scores well above the runner-up, preventing ambiguous captures from being arbitrarily pinned to a near-tie winner. Memories that fell to General by default are now reconsidered when you rename or re-describe a Topic, so improving a Topic's description actually pulls in relevant General members. Editing an auto-assigned Memory also re-runs the assignment check so the topic stays current with the new content. The Topic description placeholder in both Personal Space and Shared Space settings has been reworded to "What belongs here — new Memories are matched to it by similarity" — removing the inaccurate "AI classifier" framing.