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/index.html

@@ -921,7 +921,12 @@
     </div>
   </nav>
 
-  <main class="chat-container welcome-active" id="chat-container" role="region" aria-label="Chat area" aria-busy="false">
+  <main class="chat-container welcome-active" id="chat-container" aria-label="Chat area" aria-busy="false">
+    <!-- Persistent page heading for assistive tech. Visually hidden so it
+         never affects layout, but always present inside the main landmark
+         (the sidebar that shows the visible brand is hidden off-canvas on
+         mobile) so the page always exposes a single level-1 heading. -->
+    <h1 class="a11y-visually-hidden">Odysseus</h1>
     <div class="chat-top-bar">
       <button type="button" class="incognito-indicator" id="incognito-indicator" title="Nobody mode active — click to deactivate" style="display:none;"><svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><path d="M1 12s4-8 11-8 11 8 11 8-4 8-11 8-11-8-11-8z"/><line x1="8" y1="16" x2="16" y2="8"/><line x1="8" y1="8" x2="16" y2="16"/></svg></button>
       <div class="chat-meta-overlay"><span id="current-meta">Odysseus Chat</span><span id="current-meta-count" class="chat-meta-count" aria-hidden="true"></span><span id="session-cost-display" class="session-cost-display" style="display:none;"></span><span class="export-dropdown-wrap" id="export-dropdown-wrap"><button type="button" class="export-dl-btn" id="export-dl-btn" title="More"><svg width="13" height="13" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2.5" stroke-linecap="round" stroke-linejoin="round"><polyline points="6 9 12 15 18 9"/></svg></button><div class="export-dropdown-menu" id="export-dropdown-menu"><div class="export-dropdown-item" id="export-rename-btn"><span class="dropdown-icon"><svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><path d="M17 3a2.83 2.83 0 1 1 4 4L7.5 20.5 2 22l1.5-5.5Z"/></svg></span><span>Rename</span></div><div class="export-dropdown-item" id="export-copy-btn"><span class="dropdown-icon"><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="9" y="9" width="13" height="13" rx="2"/><path d="M5 15H4a2 2 0 0 1-2-2V4a2 2 0 0 1 2-2h9a2 2 0 0 1 2 2v1"/></svg></span><span>Copy Chat</span></div><div class="export-dropdown-item" id="export-pdf-btn"><span class="dropdown-icon"><svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><path d="M14 2H6a2 2 0 0 0-2 2v16a2 2 0 0 0 2 2h12a2 2 0 0 0 2-2V8z"/><polyline points="14 2 14 8 20 8"/><path d="M9 15v-2h2a1.5 1.5 0 0 1 0 3H9z"/></svg></span><span>PDF</span></div><div class="export-dropdown-item" id="export-doc-btn"><span class="dropdown-icon"><svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><path d="M14 2H6a2 2 0 0 0-2 2v16a2 2 0 0 0 2 2h12a2 2 0 0 0 2-2V8z"/><polyline points="14 2 14 8 20 8"/><line x1="16" y1="13" x2="8" y2="13"/><line x1="16" y1="17" x2="8" y2="17"/><polyline points="10 9 9 9 8 9"/></svg></span><span>Save to Documents</span></div></div></span></div>    </div>
@@ -2254,6 +2259,7 @@
 <script type="module" src="/static/js/assistant.js"></script>
 <script type="module" src="/static/app.js"></script>  <!-- app.js must be LAST -->
 <script type="module" src="/static/js/init.js"></script>
+<script type="module" src="/static/js/a11y.js"></script>
 <script nonce="{{CSP_NONCE}}">if('serviceWorker' in navigator){navigator.serviceWorker.register('/static/sw.js').catch(()=>{});}</script>
 </body>
 </html>