feat(invoices)!: Obsah sections, internal-only notes, unified per-page PDF header+footer

Invoices now mirror the issued-orders document model:

- New invoice_sections table (CZ/EN rich-text "Obsah") edited via the
  shared SectionsEditor, printed inline right after the items on the
  PDF. Full-replace on update, same transaction as items.
- Printed notes dropped: the notes column is removed (migration merges
  existing content into internal_notes first); the form field is now
  "Interni poznamky", never printed. Legacy payloads sending notes are
  silently stripped.
- Form cleanup: Cislo faktury and Vystavil fields removed (number lives
  in the header, issued_by auto-fills); page header title restyled to
  the orders/offers pattern (number span + status chip).
- Unified per-page PDF header for the red-accent family: shared
  buildPdfHeaderTemplate in pdf-shared (22mm logo, red heading, red
  rule) rendered by a Puppeteer headerTemplate on EVERY page of both
  invoices and issued orders (incl. the /file fallback render); body
  headers are print-hidden. htmlToPdf gained the headerTemplate option.
- Footer parity: invoices get the per-page "Vystavil + Strana X z Y"
  footer; the invoice bottom block (notice + QR/VAT recap + Prevzal)
  is break-inside: avoid so a page break can never split it.
- @page margins now match the template space (32mm top, 18mm bottom) -
  Chromium lays out by CSS @page margins, which also fixes issued
  orders' content running into the 18mm footer zone on full pages.

BREAKING CHANGE: invoices.notes column dropped (data merged into
internal_notes); deploy must run prisma migrate deploy + generate.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

prisma/migrations/20260610183000_invoice_sections_notes_internal/migration.sql

@@ -0,0 +1,32 @@
+-- Invoices: rich-text "Obsah" sections (mirrors issued_order_sections) replace
+-- the printed notes block; notes become internal-only. Existing printed notes
+-- are preserved by merging them into internal_notes before the column drop
+-- (both are Quill HTML — concatenation is valid markup).
+
+-- Preserve data: move printed notes into internal_notes
+UPDATE `invoices`
+SET `internal_notes` = CASE
+    WHEN `internal_notes` IS NULL OR `internal_notes` = '' THEN `notes`
+    ELSE CONCAT(`internal_notes`, `notes`)
+  END
+WHERE `notes` IS NOT NULL AND `notes` <> '';
+
+-- AlterTable
+ALTER TABLE `invoices` DROP COLUMN `notes`;
+
+-- CreateTable
+CREATE TABLE `invoice_sections` (
+    `id` INTEGER NOT NULL AUTO_INCREMENT,
+    `invoice_id` INTEGER NOT NULL,
+    `position` INTEGER NULL DEFAULT 0,
+    `title` VARCHAR(500) NULL,
+    `title_cz` VARCHAR(500) NULL,
+    `content` TEXT NULL,
+    `modified_at` DATETIME(0) NULL,
+
+    INDEX `invoice_sections_invoice_id`(`invoice_id`),
+    PRIMARY KEY (`id`)
+) DEFAULT CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
+
+-- AddForeignKey
+ALTER TABLE `invoice_sections` ADD CONSTRAINT `invoice_sections_fk` FOREIGN KEY (`invoice_id`) REFERENCES `invoices`(`id`) ON DELETE CASCADE ON UPDATE NO ACTION;