feat(layout): Board-Position per Lock-Button fixieren

Neuer Pin-Button (custom SVG, kein Emoji) im Board-Header sperrt die Position eines
Boards. Bei gesperrtem Board (.board.locked):
- der Drag-Handle wird per CSS ausgeblendet (Flos Wunsch: Handle weg statt nur inaktiv),
- ein zweiter Guard in drag.js onDown verweigert zusaetzlich jeden Drag.
Schuetzt vor versehentlichem Verschieben (ergaenzt den 3px-Bewegungs-Schwellwert). locked
wird wie blurred persistiert, im Export/Import durchgereicht und mit ins Trash-Board geklont.
i18n DE/EN ergaenzt.

CHANGELOG.md

@@ -12,7 +12,7 @@ All notable changes per version. Format based on [Keep a Changelog](https://keep
 - **Command Palette (Ctrl+K)** — Overlay that live-filters bookmarks (title and URL) and board names from the keyboard. Arrow-key navigation, Enter opens the match, Escape closes. Read-only navigation, separate from the web search bar. Combobox/listbox ARIA pattern with focus trap and focus return. New DE/EN i18n strings.
 - **Trash** — Deleted bookmarks and boards move to a 30-day trash instead of vanishing. Restore or permanently remove them from a new Settings section; entries older than 30 days are cleaned up automatically. Stored under its own storage key with a hard size cap so it cannot exhaust the storage quota.
 - **Quick Save** — A global keyboard shortcut (default Alt+Shift+S, configurable in the browser shortcut settings) saves the current tab into a fixed Inbox board from any page. Backed by a background worker (service worker on Chrome/Opera, event page on Firefox) that appends to a dedicated pending queue, which the dashboard drains into the Inbox — separate write domains, so a save can never clobber the boards. A badge confirms the save, and open dashboard tabs sync the new bookmark live via a storage-change listener.
-- **Free layout (bonus)** — Boards can be dragged to free positions via a drag handle, persisted per board. Positions are clamped back into view when the window shrinks, and the layout falls back to a stacked column on small screens.
+- **Free layout (bonus)** — Boards can be dragged to free positions via a drag handle, persisted per board. Positions are clamped back into view when the window shrinks, and the layout falls back to a stacked column on small screens. Each board can be pinned with a lock button: a locked board cannot be moved (its drag handle is hidden), preventing accidental repositioning. A drag only counts past a small movement threshold, so a mere click on the handle never shifts a board.
 
 ### Changed
 - The bookmark- and board-delete paths no longer remove entries immediately; deletions now route through the trash.