fix(web): UI polish — select placeholder, locked-field note, list overflow, sidebar toggle, heading wrap (#73)

Five small design/layout nits from the UI sweep:

- form.selectPlaceholder "— select —" → "Select…" / "Välj…", matching
  the affordance style of every other placeholder (Filter…, Search…).
- FieldForm in edit mode now explains its locked controls with a muted
  fields.lockedNote caption ("Key and type can't be changed after
  creation.") instead of leaving four silently disabled inputs.
- FieldList rows truncate long labels (min-w-0 on the row button +
  truncate on the label, shrink-0 on the badge and required marker)
  instead of overflowing the 20rem column.
- The sidebar collapse toggle is hidden on narrow viewports (hidden
  md:flex) instead of rendered permanently disabled/grayed — the rail
  is forced collapsed there anyway.
- PageTitle gains text-balance so long titles wrap evenly.

Closes #73

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/src/fields/field-form.tsx

@@ -110,6 +110,8 @@ export function FieldForm({
         )}
       </div>
 
+      {isEdit && <p className="text-xs text-muted-foreground">{t("fields.lockedNote")}</p>}
+
       <div className="space-y-1">
         <Label htmlFor="field-key">{t("fields.key")}</Label>
         <Input