Add signals library, restructure panel scripts, and enhance accessibility - #971

Group the scripts in `services`, `components`, `views`, `models` and `utils`,
with a single entry point (`index.ts`) that registers what the panel starts.
`app` no longer imports any component, so there are no import cycles.

Keep the state that was read from the DOM in signals, with models for the
pages tree, the calendar and the updater, and move each view to a function
started only when the page contains its element. Share the inputs base class,
the dropdown list of the select and tags inputs, the `post()` request helper
with the CSRF token and the synchronization of the file inputs.

Split the longest methods, fix the quirks found in the select and tags inputs
(stale filters, selectors broken by special characters, empty tags), document
the methods with TSDoc and replace every explicit `any` with a type.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

The panel already relies on features that are newer than `requestSubmit()`
(`:has()`, `SubmitEvent.submitter`, ARIA reflection), so the native method is
available in every browser it supports.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

Open a menu with Enter or Space and focus its first item, choose an item
with the keyboard giving the focus back to the button, close the menu with
Tab and jump to an item by typing its first letters. Follow the ARIA menu
button pattern with `aria-haspopup`, `role="menu"` and `role="menuitem"`,
and set up the buttons created after the page is loaded.

Do not move the focus to the editor when the button of the toolbar menu is
clicked, otherwise the arrow keys moved the cursor of the editor instead of
the focus in the menu.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

Keep the focus on the select input after choosing an item or closing the
list with Escape, and on the date input after Escape or Backspace, so that
Tab continues from the field and the calendar can be opened again with the
arrow keys, Enter, Space or a click. Detect the comma of the tags input with
`event.key` instead of the layout-dependent `event.code`, with the keys
configurable through the `addKeys` option.

Set `aria-expanded` on the toggles of the sections and of the pages tree,
and give the thumbnails of the dropdown lists an empty `alt`.

While the sidebar is open on small screens, make the page behind it inert,
close it when its backdrop is clicked or when the window grows to the
desktop layout, and lock the scroll of the page only below that breakpoint.
Lock the scroll of the page behind an open modal too.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

Turn the actions that clear a date and generate a slug into buttons, so
that they can be reached and activated with the keyboard, with a visible
focus ring. Name the generate button and the search inputs of the pages and
files lists with an `aria-label`, since the placeholder is not a label.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

The original code tried to strip the seconds with a string pattern that
never matched, so the value always included them. Restore that behavior
instead of removing the seconds only when they are zero.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>