fix(email-viewer): stop styled emails gluing to the corner

[shukiv]

Problem

The read-view zeroes the iframe body padding whenever an email contains a <style> tag (hasOwnLayout = hasStyleTag && !isWordHtml), assuming such mails bring their own layout. But many transactional emails ship a <style> block only for web fonts and have no outer gutter of their own — so their content renders flush against the top-left corner.

Example: a Namecheap order-summary receipt (has a <style> block, wrapper is a plain <div>, order table is width="95%", no background) rendered glued to the edge.

Fix

A <style> tag alone is too weak a signal. Only drop the default gutter when the email paints a full-bleed background canvas — a width:100% element carrying a background colour — which is the one case where our padding shows as an ugly frame around the email's own background. Otherwise keep the 1rem 1.25rem gutter.

Testing

• Namecheap receipt (<style> + no full-bleed canvas): now gets the gutter ✅
• Marketing emails with a width:100% bgcolor canvas: still edge-to-edge (no white frame) ✅
• Bare HTML / Word HTML: unchanged (already padded)
• tsc clean; lint clean

[rathlinus]

not sure, i want this as a setting i think

[shukiv]

Makes sense. Two ways to slice it:

1. Keep the refined detection as the default + add an override setting — e.g. Message spacing: Auto / Always / Edge-to-edge, Auto = this PR's heuristic. Works out of the box, power users can force either extreme.

2. Drop the heuristic, plain toggle — "Add padding around message content" (default on). Simpler, but full-bleed newsletters with a coloured canvas get a white frame when it's on.

I'd lean to (1) with Auto as default. Happy to build whichever you prefer — which do you want?