Improve accessibility across core flows (#86)

First incremental pass at issue #86, focused on the universal entry
points and primary navigation. All changes verified in-browser with the
axe-core engine (0 violations on the surfaces below) plus manual keyboard
testing, on both desktop (1280px) and mobile (390px).

Login / first-run setup (static/login.html)
- Add a real <h1>, wrap content in <main> + <footer> landmarks.
- Mark the decorative boat SVG aria-hidden.
- Errors now use role="alert" so screen readers announce them.
- "Remember me" checkbox is keyboard-focusable (was display:none) with an
  accessible name and a focus ring; dynamic 2FA field gets a linked label.
- Darken the brand-red submit button so white text clears WCAG AA 4.5:1
  (was ~3.2:1); add visible :focus-visible rings.

App shell (static/index.html, static/style.css)
- Remove invalid role="region" from the <main> chat container (it was
  overriding the implicit main landmark).
- Add a persistent, visually-hidden <h1> inside <main> so the page always
  exposes one logical level-1 heading — works even on mobile where the
  sidebar (with the visible brand) is hidden off-canvas.
- Add a reusable .a11y-visually-hidden utility.
- Raise chat-title, model-picker, settings-helper and notes text contrast
  above 4.5:1 (were 2.8-3.9:1).

Keyboard nav + dialogs (static/js/a11y.js - new)
- Make the click-only <div> sidebar navigation (New Chat, Search, Brain,
  Calendar, Compare, Cookbook, Deep Research, Gallery, Library, Notes,
  Tasks, Theme, account) focusable and Enter/Space-activatable, announced
  as buttons (skipping role=button where a nested control would create a
  nested-interactive violation). Visible focus ring reused from existing
  .list-item:focus-visible.
- Upgrade modals (.modal-content and the docked .notes-pane) to labelled
  role="dialog" + aria-modal, and normalise their title to heading level 2
  so heading order stays valid. A MutationObserver covers runtime-rendered
  rows and modals.

Decorative background canvases (static/js/theme.js)
- Mark all 7 bg-effect canvases aria-hidden.

Notes & Tasks (static/js/notes.js, static/js/tasks.js)
- Label the icon-only Note/To-do toggle pills (fixes a critical
  button-name issue) and track aria-pressed state.
- Improve Notes header-button + empty-state contrast.
- Give the Tasks sort <select> an accessible name (fixes a critical
  select-name issue).

Remaining data-dense tool modals (Tasks cards, Calendar, Gallery, Email,
Cookbook, Compare, Deep Research) still have muted-text contrast to polish
and are the next incremental step, per the issue's own guidance.

static/js/notes.js

@@ -1118,11 +1118,11 @@ export function openPanel() {
     <div class="notes-pane-header">
       <h4 class="notes-pane-title"><svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" style="vertical-align:-2.5px;margin-right:6px"><path d="M5 3h10l4 4v14H5z"/><path d="M15 3v5h5"/><path d="M8 17.5 15.5 10l2.5 2.5L10.5 20H8z"/></svg>Notes</h4>
       <span style="flex:1"></span>
-      <button id="notes-archive-toggle" class="doc-action-icon-btn notes-header-text-btn" title="View archive" style="opacity:0.6;gap:5px;">
+      <button id="notes-archive-toggle" class="doc-action-icon-btn notes-header-text-btn" title="View archive" style="opacity:0.8;gap:5px;">
         <svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><rect x="2" y="3" width="20" height="5" rx="1"/><path d="M4 8v11a2 2 0 002 2h12a2 2 0 002-2V8"/><path d="M10 12h4"/></svg>
         <span class="notes-header-btn-label">Archive</span>
       </button>
-      <button id="notes-view-toggle" class="doc-action-icon-btn notes-header-text-btn" title="Toggle view" style="opacity:0.6;gap:5px;">
+      <button id="notes-view-toggle" class="doc-action-icon-btn notes-header-text-btn" title="Toggle view" style="opacity:0.8;gap:5px;">
         <svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><rect x="3" y="3" width="7" height="7"/><rect x="14" y="3" width="7" height="7"/><rect x="3" y="14" width="7" height="7"/><rect x="14" y="14" width="7" height="7"/></svg>
         <span class="notes-header-btn-label">Toggle</span>
       </button>
@@ -1214,7 +1214,7 @@ export function openPanel() {
     const syncArchiveBtn = () => {
       archiveBtn.classList.toggle('active', _showingArchived);
       archiveBtn.title = _showingArchived ? 'Exit archive' : 'View archive';
-      archiveBtn.style.opacity = _showingArchived ? '1' : '0.6';
+      archiveBtn.style.opacity = _showingArchived ? '1' : '0.8';
       // Swap to an X while in archive view so it doubles as a close-back-
       // to-active-notes toggle.
       archiveBtn.innerHTML = _showingArchived ? CLOSE_ICON : ARCHIVE_ICON;
@@ -2022,12 +2022,12 @@ function _renderQuickAdd(body) {
   // drawing happens in the expanded form). The pill that's active steers
   // both the placeholder and the type the form opens in.
   wrap.innerHTML = `
-    <div class="notes-quick-type-seg is-todo" role="group">
-      <button type="button" class="notes-quick-type-pill" data-type="note">
-        <svg width="13" height="13" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><line x1="4" y1="6" x2="20" y2="6"/><line x1="4" y1="12" x2="20" y2="12"/><line x1="4" y1="18" x2="14" y2="18"/></svg>
+    <div class="notes-quick-type-seg is-todo" role="group" aria-label="New item type">
+      <button type="button" class="notes-quick-type-pill" data-type="note" aria-label="Note" aria-pressed="false" title="Note">
+        <svg width="13" height="13" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><line x1="4" y1="6" x2="20" y2="6"/><line x1="4" y1="12" x2="20" y2="12"/><line x1="4" y1="18" x2="14" y2="18"/></svg>
       </button>
-      <button type="button" class="notes-quick-type-pill active" data-type="todo">
-        <svg width="13" height="13" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><polyline points="9 11 12 14 22 4"/><path d="M21 12v7a2 2 0 0 1-2 2H5a2 2 0 0 1-2-2V5a2 2 0 0 1 2-2h11"/></svg>
+      <button type="button" class="notes-quick-type-pill active" data-type="todo" aria-label="To-do" aria-pressed="true" title="To-do">
+        <svg width="13" height="13" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><polyline points="9 11 12 14 22 4"/><path d="M21 12v7a2 2 0 0 1-2 2H5a2 2 0 0 1-2-2V5a2 2 0 0 1 2-2h11"/></svg>
       </button>
     </div>
     <input type="text" class="notes-quick-input" placeholder="Add a to-do…" />
@@ -2046,7 +2046,9 @@ function _renderQuickAdd(body) {
     seg.classList.toggle('is-todo', t === 'todo');
     seg.classList.toggle('is-note', t === 'note');
     seg.querySelectorAll('.notes-quick-type-pill').forEach(p => {
-      p.classList.toggle('active', p.dataset.type === t);
+      const on = p.dataset.type === t;
+      p.classList.toggle('active', on);
+      p.setAttribute('aria-pressed', on ? 'true' : 'false');
     });
     input.placeholder = t === 'note' ? 'Add a note…' : 'Add a to-do…';
   };