Add fixed/floating schedule mode to recurring task templates

Fixed mode keeps the existing behavior: next_due always advances from
the previous scheduled date, regardless of when a task is actually
completed. Floating mode instead computes the next occurrence from
the completion date of the current task, so early/late completion
shifts the whole schedule.

- task_templates gains schedule_mode ('fixed'|'floating', default
  'fixed') and pending_task_id, tracking the open task spawned from a
  floating template so the hourly sweep doesn't respawn duplicates
  before it's resolved.
- scheduler.js: floating templates set pending_task_id instead of
  advancing next_due at spawn time.
- tasks.js /resolve: floating templates recompute next_due from
  completed_at once the task is marked fixed (not temporary_fix,
  since that isn't really done yet); pending_task_id is always
  cleared on fixed resolution so a stale flag can't wedge the
  template if the mode changed mid-cycle.
- addInterval/isoDate pulled out of scheduler.js into a shared
  lib/recurrence.js used by both the scheduler and the resolve route.
- Recurring.tsx: schedule mode toggle in the template form, and the
  list shows "awaiting completion" instead of a stale next-due date
  for floating templates with an open occurrence.

frontend/src/pages/Recurring.tsx

@@ -1,6 +1,6 @@
 import { useCallback, useEffect, useMemo, useState } from 'react'
 import { Plus, X, Play } from 'lucide-react'
-import type { Template, Category, Location, Asset, Priority } from '../types'
+import type { Template, Category, Location, Asset, Priority, ScheduleMode } from '../types'
 import { PRIORITIES, PRIORITY_LABELS, can } from '../types'
 import { fetchTemplates, createTemplate, updateTemplate, runDueTemplates, fetchLocations, fetchAssets } from '../api'
 import { useAuth } from '../components/AuthGate'
@@ -20,12 +20,14 @@ interface TemplateForm {
   next_due: string
   template_notes: string
   active: boolean
+  schedule_mode: ScheduleMode
   assignment: Assignment
 }
 
 const EMPTY: TemplateForm = {
   title: '', description: '', location_id: '', asset_id: '', priority: 'medium', unusable: false,
   interval_value: 1, interval_unit: 'months', next_due: '', template_notes: '', active: true,
+  schedule_mode: 'fixed',
   assignment: { assigned_type: 'staff', assigned_to: null, assigned_to_name: null, contractor_id: null },
 }
 
@@ -78,6 +80,7 @@ export default function Recurring() {
         next_due: form.next_due,
         template_notes: form.template_notes,
         active: form.active,
+        schedule_mode: form.schedule_mode,
         assigned_type: form.assignment.assigned_type,
         assigned_to: form.assignment.assigned_to,
         assigned_to_name: form.assignment.assigned_to_name,
@@ -124,6 +127,7 @@ export default function Recurring() {
                 location_id: t.location_id, asset_id: t.asset_id || '', priority: t.priority,
                 unusable: t.unusable, interval_value: t.interval_value, interval_unit: t.interval_unit,
                 next_due: t.next_due.slice(0, 10), template_notes: t.template_notes, active: t.active,
+                schedule_mode: t.schedule_mode,
                 assignment: {
                   assigned_type: t.assigned_type, assigned_to: t.assigned_to,
                   assigned_to_name: t.assigned_to_name, contractor_id: t.contractor_id,
@@ -133,8 +137,12 @@ export default function Recurring() {
                 <td>{t.location_name}</td>
                 <td>{t.asset_name || '—'}</td>
                 <td><PriorityBadge priority={t.priority} /></td>
-                <td>{t.interval_value} {t.interval_unit}</td>
-                <td className={new Date(t.next_due) <= new Date() ? 'overdue' : ''}>{formatDate(t.next_due)}</td>
+                <td>{t.interval_value} {t.interval_unit}{t.schedule_mode === 'floating' && <span className="field-hint" style={{ display: 'block' }}>after completion</span>}</td>
+                <td className={!t.pending_task_id && new Date(t.next_due) <= new Date() ? 'overdue' : ''}>
+                  {t.schedule_mode === 'floating' && t.pending_task_id
+                    ? <span className="badge badge-outline">awaiting completion</span>
+                    : formatDate(t.next_due)}
+                </td>
                 <td>{t.assigned_type === 'contractor' ? (t.contractor_name || '—') : (t.assigned_to_name || 'Unassigned')}</td>
                 <td>{!t.active && <span className="badge badge-outline">paused</span>}</td>
               </tr>
@@ -187,6 +195,22 @@ export default function Recurring() {
                 ))}
               </div>
             </div>
+            <div className="field">
+              <label>Schedule</label>
+              <div className="chip-bar" style={{ marginBottom: 6 }}>
+                <button type="button" className={`chip ${form.schedule_mode === 'fixed' ? 'active' : ''}`} onClick={() => setForm({ ...form, schedule_mode: 'fixed' })}>
+                  Fixed
+                </button>
+                <button type="button" className={`chip ${form.schedule_mode === 'floating' ? 'active' : ''}`} onClick={() => setForm({ ...form, schedule_mode: 'floating' })}>
+                  Floating
+                </button>
+              </div>
+              <div className="field-hint">
+                {form.schedule_mode === 'fixed'
+                  ? 'Always due on the same recurring date, regardless of when each occurrence is actually completed.'
+                  : 'The next occurrence is due one interval after each task is marked fixed — late or early completion shifts the whole schedule.'}
+              </div>
+            </div>
             <div className="field-row">
               <div className="field"><label>Repeat every</label>
                 <input type="number" min={1} value={form.interval_value} onChange={e => setForm({ ...form, interval_value: parseInt(e.target.value) || 1 })} />