v1.2.0 — command palette, cross-app clipboard, autosave, a11y

Since v1.1.0 (103 commits):

• Command palette everywhere: Ctrl+K across all three apps over a unified GioAction registry, with Ctrl+? shortcut dialogs
• Cross-app clipboard: copy/paste a styled selection between Letters, Tables, and Decks via a shared fragment format
• Autosave + crash recovery: configurable autosave, atomic saves, and a close/quit guard in all three apps
• Accessibility: cells and slide objects are now real AT-SPI nodes (virtual a11y children)
• Tables: fill handle (drag-to-copy, series detection, relative formula shifting), named ranges, row filtering, print area, conditional formatting, sheet rename/delete/reorder
• Letters: footnotes, ODT read/write, page geometry + font family + line spacing round-trips, CommonMark round-trip improvements (630 → 651/652)
• Decks: object inspector, presenter pill, fit-to-viewport canvas, master slides read from PPTX, live slide thumbnails
• i18n wiring: gettext domain, POT extraction
• Flathub prep: enriched metainfo + flathub-layout manifests
• Dependency bumps (quick-xml 0.41, calamine 0.36, pulldown-cmark 0.13, zip v8) and clippy warning cleanup workspace-wide

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