feat(notifications): topic-only input (drop URL validation)

The Settings field is now a plain topic name (e.g. `my-secret-topic`)
instead of a full URL. The transport always posts to
`https://ntfy.sh/<topic>` (URL-encoded), and the only server-side check
is "non-empty when enabled". Removes the user-visible
"string does not match the expected pattern" error people hit when
typing a bare topic.

- packages/core/src/notifications/ntfy.ts: drop validateTopicUrl;
  add buildNtfyUrl(topic) + exported NTFY_BASE_URL.
- packages/core/src/notifications/types.ts, config.ts: rename
  topicUrl -> topic; update docs.
- packages/api/src/routes/notifications.ts: only validates non-empty
  topic when enabled. Also fixes a latent bug where notifySubagents
  was dropped on every PUT (was not passed to normalizeNtfyConfig).
- packages/frontend/src/lib/components/SettingsPanel.svelte: relabel
  field "Topic URL" -> "Topic"; placeholder "your-secret-topic";
  updated helper copy.
- Tests updated: rewrote validateTopicUrl coverage as buildNtfyUrl
  coverage + proof that previously-rejected topics (dots, spaces,
  unicode, "Any Topic Whatsoever") now POST cleanly.
- HANDOFF.md: added a short "topic-only input" section.

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