feat: installable offline app, plus export and import

Two things that were deliberately left out of 2.0.0.

Export writes every stored day to a JSON file. Import reads one back, and
only ever adds: a task whose id is already there is left alone, so
importing the same file twice does nothing and importing into a list
you're using can't lose work. That is also why it needs no confirmation
dialog. The trade is that import restores rather than reverts.

Both sit in a quiet line under the task list rather than the header, since
they get used about twice a year and the header is what you look at all
day.

For the PWA half, the service worker is about fifty lines with no
dependency, because the strategy falls out of how Vite builds. Documents
go network first and fall back to cache, so a deploy is picked up as soon
as you're online and nobody ends up stuck on an old build. Fingerprinted
assets go cache first and are kept, since their names change when their
contents do. No build-time asset manifest needed.

Icons are SVG in the manifest, which stays sharp at any size and costs
about a kilobyte, plus one 180px PNG because iOS wants a raster
apple-touch-icon. Two theme-color metas so the phone status bar follows
the theme, and safe-area padding on the header, without which the header
sits under the clock once installed on an iPhone.

Verified against the production build: worker registers and claims the
page, shell and assets land in cache, and with the server stopped the app
still loads, adds a task and persists it.

src/App.svelte

@@ -11,6 +11,7 @@
   import * as taskOps from './lib/tasks.js';
   import { createUndoStack, applyUndo } from './lib/undo.js';
   import { shouldHandleUndo } from './lib/shortcuts.js';
+  import { buildExport, serialize, parseImport, mergeImport } from './lib/backup.js';
 
   const storage = createStorage();
   const undoStack = createUndoStack();
@@ -83,6 +84,63 @@
     darkMode = !darkMode;
   }
 
+  let fileInput;
+  let status = $state('');
+  let statusIsError = $state(false);
+  let statusTimer = null;
+
+  function showStatus(message, isError = false) {
+    status = message;
+    statusIsError = isError;
+    clearTimeout(statusTimer);
+    statusTimer = setTimeout(() => { status = ''; }, STATUS_MS);
+  }
+
+  function exportTasks() {
+    const text = serialize(buildExport(storage));
+    const url = URL.createObjectURL(new Blob([text], { type: 'application/json' }));
+
+    const link = document.createElement('a');
+    link.href = url;
+    link.download = `negotium-${todayKey}.json`;
+    link.click();
+    URL.revokeObjectURL(url);
+
+    showStatus('Exported.');
+  }
+
+  async function importTasks(event) {
+    const file = event.target.files?.[0];
+    // Reset first, so picking the same file twice still fires a change event.
+    event.target.value = '';
+    if (!file) return;
+
+    let parsed;
+    try {
+      parsed = parseImport(await file.text());
+    } catch {
+      showStatus("That file couldn't be read.", true);
+      return;
+    }
+
+    if (!parsed.ok) {
+      showStatus(parsed.error, true);
+      return;
+    }
+
+    const { imported, duplicates, days } = mergeImport(storage, parsed);
+    tasks = storage.loadTasks(selectedKey);
+
+    if (imported === 0) {
+      showStatus(duplicates > 0 ? 'Already up to date.' : 'Nothing to import.');
+      return;
+    }
+
+    const taskWord = imported === 1 ? 'task' : 'tasks';
+    const dayWord = days === 1 ? 'day' : 'days';
+    showStatus(`Imported ${imported} ${taskWord} across ${days} ${dayWord}.`);
+  }
+
   /** Scoped to the input. Previously this also sat on window, so Enter while a
    *  task was focused would toggle that task *and* add whatever was in the
    *  input. */
@@ -126,6 +184,7 @@
   const DRAG_THRESHOLD_PX = 8;
   const LONG_PRESS_MS = 400;
   const SETTLE_MS = 240;
+  const STATUS_MS = 4000;
 
   let drag = null;
 
@@ -534,6 +593,22 @@
           {/if}
         {/key}
       </div>
+
+      <footer class="data-footer">
+        <button class="data-link" onclick={exportTasks}>Export</button>
+        <span class="data-sep" aria-hidden="true">·</span>
+        <button class="data-link" onclick={() => fileInput.click()}>Import</button>
+        <input
+          bind:this={fileInput}
+          type="file"
+          accept="application/json,.json"
+          class="visually-hidden"
+          onchange={importTasks}
+        />
+        <span class="data-status" class:error={statusIsError} role="status" aria-live="polite">
+          {status}
+        </span>
+      </footer>
     </div>
   </main>
 </div>