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/styles/editor.css

@@ -391,6 +391,50 @@ body {
   min-height: 100%;
 }
 
+/* ==========================================================================
+   CANVAS = PUBLISHED BASE RESET
+   Published pages and Preview ship a reset in the document head
+   (utils/css-reset.ts CSS_RESET: `*{margin:0;padding:0}`, img{display:block},
+   a{color:inherit}, body line-height 1.6 / colour #1f2937 / 16px). The editor
+   canvas lives inside the editor document, whose own body is 13px editor
+   chrome with no such reset, so content looked different (default h1-h6/p/ul
+   margins, `normal` line-height, 13px inherited text). This block gives the
+   canvas FRAME -- the <body> stand-in -- and everything inside it the same
+   defaults.
+   Rules:
+   - Every selector is ZERO specificity (:where). The published reset is `*`
+     (specificity 0), so component inline styles and class rules beat it; a
+     plain `.canvas-device-frame *` (0,1,0) would instead beat component
+     <style> class rules and editor chrome rules. Do NOT remove the :where().
+   - Scoped to the frame, never body/:root, so panels, toolbars and Craft
+     overlays (all rendered outside the frame) are untouched. Editor-only
+     elements inside the frame style themselves with classes / inline styles,
+     which outrank this block.
+   - Declarations come from utils/css-reset.ts; css-reset.test.ts fails if
+     this block and that module drift apart. The font-family / colour tokens
+     are layered on top by CanvasTokenStyle exactly as the published head
+     layers them over the reset.
+   ========================================================================== */
+.canvas-device-frame {
+  font-size: 16px; /* UA default a published <body> gets; editor body is 13px */
+  font-family: 'Inter',-apple-system,BlinkMacSystemFont,'Segoe UI',Roboto,sans-serif;
+  line-height:1.6;
+  color:#1f2937;
+  -webkit-font-smoothing:antialiased;
+}
+:where(.canvas-device-frame) :where(*),
+:where(.canvas-device-frame) ::before,
+:where(.canvas-device-frame) ::after {
+  box-sizing:border-box;margin:0;padding:0;
+}
+:where(.canvas-device-frame) :where(img) {
+  max-width:100%;height:auto;display:block;
+}
+:where(.canvas-device-frame) :where(a) {
+  color:inherit;
+}
+/* END CANVAS = PUBLISHED BASE RESET */
+
 /* --------------------------------------------------------------------------
    No Selection Message
    -------------------------------------------------------------------------- */