feat(email): introduce React Email templates and a shared Oblidog layout #478

Goal

Establish a reusable email design system and preview/export workflow so scheduled reports and transactional emails can be redesigned consistently.

Scope

• Add a React Email package/workspace with reproducible locked dependencies and preview/export commands.
• Build shared branded layout, heading, section, summary, obligation-row, button and footer components.
• Use readable typography, restrained Oblidog branding and mobile-friendly layouts; avoid wide dashboard-like tables.
• Define and demonstrate the build-time HTML export / Python Jinja integration for dynamic collections, conditions and escaping. A static HTML export alone must not freeze report data.
• Export templates during the build or commit generated artifacts with a deterministic freshness check; document the chosen approach.
• Keep report queries, business rules, scheduling and SMTP delivery in Python. Production sending must not require a Node service or external rendering API.
• Add synthetic preview fixtures for empty, normal, long-name, long-list, multi-ledger and multi-currency data.

Acceptance criteria

• Preview and export are reproducible through documented commands.
• Python safely renders variable-length collections and conditional sections; user-supplied content is escaped.
• Generated HTML is available in backend images and applicable deployment packaging.
• Shared layout is readable at mobile and desktop widths, including images-disabled mode and common email-client constraints.
• Representative rendered previews are visually reviewed.
• Plain-text delivery remains supported.

Out of scope

Scheduling/preferences changes (#139), delivery retry semantics (#375), changes to report selection or calculations.

Priority

High product value; prerequisite for the report redesign issues.

Upstream references

fastapi/full-stack-fastapi-template PRs fastapi/full-stack-fastapi-template#2421 and fastapi/full-stack-fastapi-template#2451. Adapt the integration to Oblidog's dynamic reports rather than copying only static examples.

Follow-up issues

• feat(reports): redesign the daily digest with React Email #479 — daily digest redesign (first report).
• feat(reports): redesign the weekly monthly overview with React Email #480 — weekly overview redesign.
• refactor(email): migrate transactional emails to the shared React Email layout #481 — transactional email migration.
• test(email): replace Mailcatcher with Mailpit and simplify email E2E helpers #477 — local Mailpit inspection and E2E helpers (can proceed independently).