feat(calendar): enable calendar sync for Viktiga datum (#1853)

Turns on the calendar extension (built Feb 2026, stripped in the 2026-03-02 production readiness deploy, never re-enabled): ICS feed settings, calendar workspace, subscribe button on the Viktiga datum page.

Hardening before first real use: feed serve route now requires the creator to still be a company member (offboarding stops the feed); stable pagination (due_date + id, dedupe) on feed queries; fetches inside the logged try block; invoice events limited to sent/paid/partially_paid/overdue; event UIDs rebranded to accounted.se while zero feeds exist; APP_URL fallback fails closed in production; mobile stacking for the deadlines header; settings note that Google Calendar needs default notifications on subscribed calendars; calendar workspace aligned with the design system.

Skeptic reviewed (3 refutations, all fixed) plus one compliance swarm finding (fixed). No migrations.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/extensions/_generated/workspace-map.tsx

@@ -4,6 +4,7 @@ import type { ComponentType } from 'react'
 import type { WorkspaceComponentProps } from '../workspace-registry'
 
 export const WORKSPACES: Record<string, ComponentType<WorkspaceComponentProps>> = {
+  'general/calendar': dynamic(() => import('@/components/extensions/general/CalendarWorkspace')),
   'general/enable-banking': dynamic(() => import('@/components/extensions/general/EnableBankingWorkspace')),
   'general/arcim-migration': dynamic(() => import('@/components/extensions/general/ArcimMigrationWorkspace')),
   'general/tic': dynamic(() => import('@/components/extensions/general/TicWorkspace')),