Fix #139: drop unsafe-inline from script-src and style-src (#139)

- Move all inline <script> blocks (layout head/theme, topbar dark toggle,
  foot, paste, new, history, mine, settings, admin, unlock) to external
  files under internal/web/static/. Page data reaches scripts via data-*
  attributes (data-paste-id, data-default-dark) instead of template vars.
- Replace inline onclick handlers (copy, delete, stats toggle) with
  addEventListener wiring.
- Convert inline style="" attributes to CSS utility classes; swatch
  colors are now set via CSSOM/DOM APIs instead of innerHTML strings.
- script-src/style-src are now plain 'self'; img-src data: stays for the
  SVG data-URI backgrounds. Verified with headless chromium: zero CSP
  violations on all pages in dark and light presets, theme swatches,
  admin lock, tables and paste view render correctly.

internal/web/templates/can.html

@@ -1,7 +1,7 @@
 {{template "head" .}}
 {{template "topbar" .}}
 <div class="center">
-  <div class="float can-page" style="max-width:900px; width:100%;">
+  <div class="float can-page">
     <div class="inner">
       <h1>{{.Title}} <span class="badge" title="This is a can — a bundle of pastes">can</span></h1>
       {{if .HasDescription}}<p class="sub">{{.Description}}</p>{{end}}