feat(email-viewer): message spacing setting (auto/always/edge-to-edge)

[shukiv]

Follow-up to #595, built the way you asked ("i want this as a setting").

The problem (from #595)

The reader zeroes the message-body gutter whenever an email contains a <style> tag (hasOwnLayout = hasStyleTag && !isWordHtml). Plenty of transactional mails ship a <style> block only for web fonts and have no gutter of their own, so their content renders glued to the top-left corner (a Namecheap order receipt was the reported case).

The setting

New Message Spacing option under Settings → Reading:

Mode	Behaviour
Auto (default)	Add the gutter unless the email paints a full-bleed background canvas — a width:100% element carrying a background colour — which is the one case where our gutter shows as a frame around the email's own background
Always add spacing	Always keep the gutter
Edge to edge	Never add one (previous behaviour for styled mail)

Auto keeps it working out of the box; the two explicit modes let users force either extreme. A <style> tag alone is no longer treated as "this mail brings its own layout".

Changes

• stores/settings-store.ts — MessageSpacing type, messageSpacing state (default 'auto'), added to the persist allowlist
• components/email/email-viewer.tsx — full-bleed-canvas detection drives auto; setting overrides it; added to the srcDoc memo deps
• components/settings/reading-settings.tsx — Select control (respects the existing policy lock/hide hooks)
• i18n: settings.email_behavior.message_spacing.* (en + he)

Testing

• Namecheap-style receipt (<style>, no full-bleed canvas) → Auto gives it the gutter
• Newsletter with a width:100% bgcolor canvas → Auto stays edge-to-edge (no white frame)
• Always / Edge to edge override both cases; setting persists across reload
• Bare HTML and Word/Outlook HTML unchanged
• tsc clean; lint clean (no new warnings)