v2.1.0 — crash fixes: RefCell reentrancy panics

Crash fixes only, no other functional changes since v2.0.0.

A dedicated audit for RefCell-reentrancy crashes (a borrow held across a call that synchronously reenters and takes another conflicting borrow) found and fixed real, reproducible panics in all three apps:

• Tables (#138, #139): opening any file via File → Open, single-click or arrow-key navigation onto a formula cell, double-clicking a column divider to auto-fit, and deleting a sheet via the confirmation dialog all crashed with "RefCell already borrowed".
• Letters (#140): double-clicking an existing ruler tab stop crashed the same way.
• Decks (#141): switching between two slides that both have speaker notes crashed (GtkTextBuffer's delete-then-insert emits changed twice, and the skip-guard only survived one emission) — reachable from slide add/delete/move, undo/redo, keyboard navigation, new-document, and file-open.

Install: flatpak remote-add --if-not-exists tuna-os https://tunaos.org/flatpak/tuna-os.flatpakrepo && flatpak update org.tunaos.tables org.tunaos.letters org.tunaos.decks