Add trip checklists with rule-based packing advice

Each trip gets a checklist whose items group under free-text categories
(Documents, Clothing, Toiletries, Health, Electronics, Extras first, then
any custom ones alphabetically). Items are either shared — every member
sees and can tick them, and checked_by records who — or personal to one
member, which nobody else can see or touch. Items carry an optional
quantity, drag-reorder within their category, and "Uncheck all" resets the
list for the trip home.

The "Suggestions" modal is deterministic, offline advice derived from the
trip itself (src/server/util/packing.js) — no LLM and no external calls, so
it stays unit-testable and works on a self-hosted box. Nights scale
clothing quantities, flights add liquids/power-bank/check-in, rentals add
licence + IDP, ferries add motion-sickness tablets, tropical stops add sun
cream and repellent, and the destination country picks the plug type from a
bundled ~50-country table. Every suggestion carries a short reason, and
already-added ones are keyed by suggestion_key so they can't be duplicated.

Two rules deliberately differ from the naive reading, both regression-tested:
a latitude floor stops a December trip to Bangkok being tagged cold as well
as tropical, and only a flight segment's arrival airport counts, since the
first segment's departure airport is home rather than a destination.

checklist_items is a new table, so the existing CREATE TABLE IF NOT EXISTS
path creates it on upgrade; no MIGRATIONS entry is needed and existing data
is untouched.

docs/API.md documents the full contract. 113/113 tests pass.

public/js/views/tripDetail.js

@@ -10,6 +10,7 @@ import { renderCalendar } from './calendar.js';
 import { renderMap } from './map.js';
 import { renderSummary } from './summary.js';
 import { renderCosts } from './costs.js';
+import { renderChecklist } from './checklist.js';
 import { renderCountdown } from './flipclock.js';
 import { openDayEditor } from './dayEditor.js';
 
@@ -74,7 +75,7 @@ export function renderTripDetail(container, ctx, id) {
         'div',
         { class: 'detail-grid' },
         renderMap(tctx),
-        el('div', { class: 'detail-side' }, renderSummary(tctx), renderCosts(tctx)),
+        el('div', { class: 'detail-side' }, renderSummary(tctx), renderCosts(tctx), renderChecklist(tctx)),
       ),
     );
     mount(container, page);