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/tasks.js

@@ -2401,7 +2401,7 @@ function _renderMainView() {
       <p class="memory-desc" style="position:relative;top:-4px;">Scheduled prompts and actions that run automatically. Results appear in a dedicated session.</p>
       <div class="memory-toolbar">
         <div class="memory-category-filters" style="display:flex;align-items:center;gap:6px;">
-          <select class="memory-sort-select" id="tasks-sort" style="position:relative;top:-4px;width:86px;font-size:11px;height:24px;">
+          <select class="memory-sort-select" id="tasks-sort" aria-label="Sort tasks" title="Sort tasks" style="position:relative;top:-4px;width:86px;font-size:11px;height:24px;">
             <option value="recent">Recent</option>
             <option value="name">A–Z</option>
             <option value="status">Status</option>