docs: add CHANGELOG

CHANGELOG.md

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+# Changelog
+
+## [0.1.0] — 2026-05-25 · MVP
+
+### Stack
+- Vite + Svelte 5 (runes mode) + TypeScript
+- TipTap (individual extensions, no starter-kit)
+- odf-kit for ODT generation
+- No backend — fully client-side
+
+### Implemented
+- WYSIWYG editor with TipTap
+  - Bold, Italic, Underline
+  - Headings H1–H3
+  - Undo / Redo (History extension)
+  - Placeholder text on empty document
+- Toolbar with active-state highlighting per button
+  - Reactivity via `tick` counter (Svelte 5 same-reference workaround)
+- Fixed A4 paper canvas (794 × 1123 px at 96 dpi)
+- LocalStorage auto-save (debounced 1 s, restores on reload)
+- ODT export via `tiptapToOdt()` from odf-kit
+  - Filename derived from first H1/H2/H3 heading, fallback `document.odt`
+- Favicon (hand-coded SVG, no external assets)
+
+### Not yet implemented
+- Lists (ordered / unordered)
+- light/dark and auto appearance mode in a settings menu
+- ODT import / open existing file
+- font size
+- Images, tables
+- DIN A4 print layout / page breaks
+- Multi-document management
+- PWA / offline support
+- i18n
+- ...
+
+---
+
+<!-- Add new entries above this line in the format: -->
+<!-- ## [x.y.z] — YYYY-MM-DD -->