refactor(webserver): migrate presentational HTML attributes to CSS - #95

Replace the HTML 3.2-era presentational attributes across the default
template with equivalent CSS classes in style.css, keeping the quirks
mode doctype untouched so the table layout renders exactly as before:

- body background= / leftmargin= / topmargin= / marginwidth= /
  marginheight= become `body.main { margin: 0; background-image: ... }`.
- background= on the header bar and the tab-frame cells become the
  navbar-cell and tab-top/left/right/bottom classes.
- bgcolor= becomes bg-white / bg-black / sep-dark / sep-light (the
  1px black and grey separator rows in the file listings).
- align= / valign= on rows and cells become al-* / va-* classes.
  Tables that relied on the legacy block-alignment behaviour of
  align="center" / align="right" (nav bar, toolbars, prefs panes,
  login box) get margin-based center-table / navbar-table classes
  instead, since CSS text-align only moves inline content.
- height= on td/th becomes h10..h200 utility classes.
- border="0" is dropped everywhere; a global `img { border: 0 }`
  rule covers linked images.

login.php keeps a private inline <style> copy of the rules it uses:
the webserver only serves style.css to a logged-in session (any other
request gets the login page HTML instead), and login.php is precisely
the page unauthenticated clients see. It previously survived this
only because the legacy attributes needed no stylesheet. index.html
links no stylesheet either, so its body uses an inline style.

Also fixes the malformed `width="100% "height="180"` attribute pair
in login.php and the stray valign= on its outer table.

Verified against a running amuleweb: before/after headless-Chromium
screenshots of all 9 pages are pixel-identical except for dynamic
content (stats graphs, server user/file counts).