fix(site-builder): editor canvas renders content with the published base reset

Published pages/Preview ship a head reset (*{margin:0;padding:0}, img block,
a inherit, body line-height 1.6 / #1f2937 / 16px); the editor canvas had none,
so h1-h6/p margins, line-height and inherited size/colour differed.

- editor.css: one commented block scoping the reset to .canvas-device-frame,
  every selector zero-specificity (:where) so inline styles / class rules win
  exactly as when published; editor chrome untouched (snapshot identical).
- tokens.ts: canvas heading/link token rules made zero-specificity too.
- utils/css-reset.ts: single source for the reset declarations (html-export
  imports it; test asserts byte-identity with the backend head and that the
  editor.css block carries the same declarations).
- scripts/e2e-canvas-parity.mjs (npm run test:browser:canvas): real editor vs
  published HTML in Firefox + Chromium, boxes within 1px + computed styles.

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

craft/src/utils/css-reset.ts

@@ -0,0 +1,21 @@
+/**
+ * The page-level base reset shared by the PUBLISHED/Preview document head
+ * (html-export.ts `wrapInDocument`, mirrored by the backend's
+ * `generateCompiledHTML` in web-files/api/site-builder.php) and the editor
+ * CANVAS (the "canvas = published reset" block in styles/editor.css).
+ *
+ * The declaration strings live here once so the two cannot drift;
+ * css-reset.test.ts asserts html-export's CSS_RESET is byte-identical to the
+ * published head text and that the editor.css block carries every one of
+ * these declarations.
+ */
+export const RESET_ALL_DECLS = 'box-sizing:border-box;margin:0;padding:0';
+export const BODY_BASE_DECLS =
+  "font-family:'Inter',-apple-system,BlinkMacSystemFont,'Segoe UI',Roboto,sans-serif;line-height:1.6;color:#1f2937;-webkit-font-smoothing:antialiased";
+export const IMG_DECLS = 'max-width:100%;height:auto;display:block';
+export const LINK_DECLS = 'color:inherit';
+
+/** Minified published reset (document head). Must stay byte-identical to the backend's `$reset`. */
+export const CSS_RESET =
+  `*,*::before,*::after{${RESET_ALL_DECLS}}html,body{overflow-x:clip}` +
+  `body{${BODY_BASE_DECLS}}img{${IMG_DECLS}}a{${LINK_DECLS}}`;