feat: replace YAML notification channel editor with form-based UI

Notification channels are now managed through a proper web form instead
of a raw YAML textarea. Any authenticated user can create channels; private
channels (owner-scoped) are hidden from other users. The user profile
channel selector becomes a tag/chip picker with a "My Channels" CRUD section.

- settings.py: add CHANNEL_TYPE_SCHEMAS for all 6 notifier types; channel
  section switches to section_mode="channels"; cards include owner/private/min_level
- configio.py: add apply_channel() and delete_channel() for per-entry CRUD
- notify.py: strip owner/private metadata before dispatching to drivers
- http.py: add GET/POST /api/0/notification_channels, PUT/DELETE /{name},
  GET /api/0/notification_channel_types; visibility helper filters private
  channels per user; PUT /api/0/users/me validates against visible channels
- settings.html: card grid with edit/delete per channel; add/edit modal
  with type dropdown and dynamically rendered type-specific fields
- profile.html: chip picker replaces checkbox list; My Channels section
  for creating/editing/deleting user-owned channels
- tests: update test_settings_sections, test_http_users_me; add
  test_notification_channels_api (16 new tests, 46 total passing)

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

hbd/server/configio.py

@@ -93,6 +93,19 @@ def apply_structured_section(data, section: str, values: dict) -> None:
         raise ValueError(f"Unknown structured section: {section!r}")
 
 
+def apply_channel(data, name: str, channel_cfg: dict) -> None:
+    """Insert or replace a single notification channel entry, preserving others."""
+    if not data.get("notification_channels"):
+        data["notification_channels"] = {}
+    data["notification_channels"][name] = channel_cfg
+
+
+def delete_channel(data, name: str) -> None:
+    """Remove a notification channel by name. No-op if not found."""
+    nc = data.get("notification_channels") or {}
+    nc.pop(name, None)
+
+
 def apply_yaml_section(data, section: str, yaml_text: str) -> None:
     """Replace the named logical section by parsing yaml_text."""
     parsed = _make_yaml().load(yaml_text)