Add in-place dashboard edit mode

A toggle on the dashboard flips into edit mode where widgets get
drag/edit/delete handles and a "+ Add widget" button per column. Most
edits no longer require touching YAML.

- Drag-drop reorder via SortableJS, persisted through a bulk-layout
  endpoint that rebuilds each column's widgets sequence in the
  yaml.Node tree.
- Inline form dialog driven by per-widget field schemas
  (admin-schemas.go) covering 25 widget types. Surgical updates apply
  changed fields to the widget node so untouched keys and comments
  survive. Falls back to the existing YAML editor for group and
  split-column (those need a recursive nested-widget UI).
- Per-column "+ Add widget" picker that opens the dialog pre-filled
  with sensible defaults for the chosen type.
- Field validation and autocomplete for weather location (Open-Meteo
  geocoding), market symbol (Yahoo Finance), and RSS feed URL
  (gofeed). Network failures soft-pass for weather/markets so the
  save isn't blocked when external APIs are unreachable.
- Picking a market suggestion auto-fills the company name field via
  per-suggestion `extra` fields.
- Edit mode persists across reloads via localStorage so saves don't
  drop the user back into view mode.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/glance/static/css/main.css

@@ -64,3 +64,4 @@
 @import "popover.css";
 @import "utils.css";
 @import "mobile.css";
+@import "edit-mode.css";