fix(a11y): P5 / G4 — add focus trap to MorningTriageModal

Phase 10a a11y audit (2026-04-29) flagged the morning triage modal as
having role="dialog" and aria-modal="true" but no focus trap, so Tab
leaks out to the sidebar/page beneath. Escape-to-close was already
wired and is preserved.

Behaviour now matches the W3C dialog pattern:

- On open, capture the invoking element (document.activeElement) and
  move focus to the first focusable inside the dialog.
- Tab cycles forward; Shift+Tab cycles backward. Wrap-around between
  first and last focusable.
- On close, restore focus to the captured invoker.
- The dialog container itself gets tabindex=-1 so it can hold focus
  if no children are focusable (e.g. during the loading state).

The focusable selector excludes aria-hidden elements and elements with
no offsetParent (display:none / visibility:hidden), so dynamic content
between loading -> tasks -> action buttons stays in sync.

Resolves: G4.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/components/MorningTriageModal.svelte

@@ -34,6 +34,11 @@
   let tooManyFlash = $state(false);
   let applying = $state(false);
   let focusHandler: (() => void) | null = null;
+  // Phase 10a a11y G4: focus-trap state. The modal must keep keyboard
+  // focus inside its bounds while open and restore focus to the
+  // invoking element on close.
+  let dialogEl = $state<HTMLDivElement | null>(null);
+  let invokerEl: HTMLElement | null = null;
 
   function todayKey(): string {
     const d = new Date();
@@ -171,14 +176,73 @@
     }
   }
 
+  // Phase 10a a11y G4: collect every focusable element inside the dialog
+  // for the focus trap. We re-query on every Tab so dynamic content
+  // (loading → tasks → buttons) stays in sync.
+  function focusableInDialog(): HTMLElement[] {
+    if (!dialogEl) return [];
+    const sel = [
+      'a[href]',
+      'button:not([disabled])',
+      'input:not([disabled])',
+      'select:not([disabled])',
+      'textarea:not([disabled])',
+      '[tabindex]:not([tabindex="-1"])',
+    ].join(',');
+    return Array.from(dialogEl.querySelectorAll<HTMLElement>(sel)).filter(
+      (el) => !el.hasAttribute('aria-hidden') && el.offsetParent !== null,
+    );
+  }
+
   function handleKeydown(e: KeyboardEvent) {
     if (!open) return;
     if (e.key === 'Escape') {
       e.preventDefault();
       skipForToday();
+      return;
+    }
+    if (e.key === 'Tab') {
+      const focusables = focusableInDialog();
+      if (focusables.length === 0) {
+        // Nothing focusable inside — keep focus on the dialog container.
+        e.preventDefault();
+        dialogEl?.focus();
+        return;
+      }
+      const first = focusables[0];
+      const last = focusables[focusables.length - 1];
+      const active = document.activeElement as HTMLElement | null;
+      if (e.shiftKey) {
+        if (active === first || !dialogEl?.contains(active)) {
+          e.preventDefault();
+          last.focus();
+        }
+      } else {
+        if (active === last || !dialogEl?.contains(active)) {
+          e.preventDefault();
+          first.focus();
+        }
+      }
     }
   }
 
+  // Phase 10a a11y G4: when the dialog opens, remember the invoker so
+  // we can return focus on close, and move focus into the dialog.
+  $effect(() => {
+    if (open) {
+      invokerEl = (document.activeElement as HTMLElement) ?? null;
+      // Wait one tick for the dialog to mount before focusing.
+      queueMicrotask(() => {
+        const focusables = focusableInDialog();
+        (focusables[0] ?? dialogEl)?.focus();
+      });
+    } else if (invokerEl) {
+      // Restore focus to the element that opened the modal.
+      try { invokerEl.focus(); } catch {}
+      invokerEl = null;
+    }
+  });
+
   onMount(() => {
     maybeShow();
     focusHandler = () => maybeShow();
@@ -194,10 +258,12 @@
 
 {#if open}
   <div
+    bind:this={dialogEl}
     class="fixed inset-0 z-50 flex items-center justify-center bg-black/50 backdrop-blur-sm animate-fade-in"
     role="dialog"
     aria-modal="true"
     aria-labelledby="triage-title"
+    tabindex="-1"
   >
     <div class="bg-bg-elevated border border-border rounded-2xl shadow-2xl max-w-[480px] w-[90vw] max-h-[80vh] flex flex-col">
       <div class="px-6 pt-6 pb-3">