Added simple editor for the text under the banner, stored HTML is sanitized. Fixes #17

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.claude/rules/backend.md

@@ -31,6 +31,7 @@ paths:
 - The auth middleware protects only `/admin*` page requests. `/api/*` is not covered by it — a server fn without `perm_check!` is public. Only the public booking form endpoints should be.
 - Plain Actix handlers must check the session themselves.
 - Files are uploaded through a multipart server function (`upload_image` in `src/backend/appearance.rs`): `#[server(input = MultipartFormData)]`, called in the browser through `Action::new_local` with a `web_sys::FormData`. The server stores a file under a name it makes itself, checks size and content, and never trusts the name sent by the browser. The multipart codec panics on a request without a boundary; `is_broken_upload` in `src/main.rs` refuses those first — extend it when adding another multipart function.
+- HTML that comes from a user and is rendered with `inner_html` (the text under the banner) must go through `sanitize_text` in `src/backend/rich_text.rs` before it is stored. Extend its allow-list together with the editor toolbar, never the other way round.
 - The logged-in `User` is stored in the cookie session under `"user"`; read it with `logged_in_user()`.
 
 ## Database access

.claude/rules/frontend.md

@@ -39,6 +39,7 @@ paths:
 
 - Every user-visible string goes through `trl("English text")`; never hard-code Czech (or any text) in a view. The English text is the key and the fallback.
 - When you add or change a key, add the Czech translation to the `cs` map in `src/locales/catalogues.rs`. This includes validation `message`s and `ApiResponse::Error` strings from the backend.
+- `trl()` reads the language from context, so in an event handler it silently returns the English key. Translate in the component body or in the view; keep a key in a signal and translate it when rendering (see `ImageUpload` in `src/pages/appearance_settings.rs`).
 - `trl()` returns a closure — use it directly as `{trl("...")}` or attribute value; call it (`trl("...")()`) when a `String` is needed.
 - Dates shown to users go through `loc_date()`, weekday names through `show_day()`.