feat(email): unify verify / reset / claim email layouts

[alejandro-runner]

Summary

Apply the visual language of the new team-invite email to the three remaining transactional emails — verification, password reset, and account claim — so every Synvya email recipients see shares the same header, button, and footer treatment.

Not a card rewrite. These flows have no subject entity to present (unlike the team invite, which shows team avatar + display name), and over-stylising reset/verify emails can read as phishing. The goal is consistency, not maximalism.

Changes

• api/src/email_service.rs — new shared basic_email_html(heading, intro, cta_label, cta_url, footer_note) helper. verification_email_html, password_reset_html, and claim_email_html now call it. All interpolated values HTML-escaped.
• Net: −69 / +61 lines (the three templates collapsed into calls against the shared helper, removed per-template duplicated styles).

Before / After

Before: left-aligned teal h1, left-aligned plain paragraph, left-aligned button with 4px radius, left-aligned copy-link, left-aligned footer. Each template duplicated the full HTML scaffold.

After: centred Synvya wordmark, centred heading (h2, 20px), centred intro paragraph, centred rounded-pill button (8px radius, 14×36 padding), centred copy-link fallback, centred muted footer.

Shared with the team-invite email: same background (#f5f5f5), same max-width (520px), same teal (#00B488), same button geometry.

Out of scope

• Team-invite email (already on this layout, landed in feat(invitations): card-style invite email with kind-0 team profile #33).
• No copy rewrites beyond minor tightening of the intro sentences.
• No new provider behaviour — SendGrid / SES / Dev impls unchanged.

Test plan

• `cargo check --workspace` clean
• `cargo clippy -p keycast_api --all-targets -- -D warnings` clean
• `cargo test -p keycast_api --lib` — 62 passed
• Staging smoke: trigger a registration (verify), a password-reset request, and an admin-preload claim; confirm each email renders the shared layout in Gmail + Apple Mail.

🤖 Generated with Claude Code

[alejandro-runner]

Added a second commit (304f509) with a card layout for the claim email:

• Extends send_claim_email with account_display_name / account_picture options.
• claim_email_html renders the invite-style card (avatar + restaurant name + "Claim your login for {email}") when kind-0 is available; otherwise falls back to the plain single-CTA layout.
• admin.rs batch claim handler fetches kind-0 for each preloaded pubkey (3s timeout, silent fallback) before sending.

Rationale: preloaded accounts are onboarded with kind-0 set up, so the card gives the recipient concrete brand context ("yes, this claim email is for my restaurant") rather than a generic "account ready" prompt. Reset/verify remain plain on purpose — no subject entity, and over-stylised password emails can read as phishing.