Save, load, and export a workbook

What it does

A File control in the toolbar (the save icon, rightmost group) opens a menu:

• Save to browser — writes the whole workbook (every cell's raw input + every cell's format) to local storage, in one slot.
• Load from browser — reads that slot back and replaces the current workbook with it exactly: cells and formats not in the saved copy are cleared, not merged.
• Import JSON file… — the same replace, from a .json file picked via a file input (for moving a workbook between browsers/machines, not just within one).
• Export JSON — downloads the same canonical JSON as Save writes, as a file.
• Export CSV — downloads the resolved grid values as CSV.
• Export XLSX — downloads a .xlsx workbook (values + formulas + cell formatting) built with exceljs, lazy-loaded only when this button is used so it never bloats the app's initial bundle.

Verified behavior

Matches Google Sheets: Google Sheets autosaves continuously to Drive, and separately offers File → Download as a one-way export (XLSX / CSV / … ) of the current sheet's values and formulas. Studio has no server, so "save" here is the equivalent local primitive: File → Save writes the ENTIRE workbook — every cell's raw input (literal or formula) and every cell's presentation format (bold, italic, color, fill, …) — to the browser's local storage, keyed by one slot ("the workbook"); File → Load reads that slot back and REPLACES the current workbook with it exactly, byte-for-byte, the same way opening a saved Sheets file replaces what's on screen. Export mirrors Sheets' Download menu: JSON is Studio's own canonical schema (not a Sheets format, so there is no Sheets ground truth to match — it exists so the saved data is inspectable/portable); CSV is the resolved (computed) grid values, comma/quote/newline-escaped per RFC 4180, matching what Sheets' "Download → csv" produces for a single sheet; XLSX carries both the resolved value AND the formula text per cell (SheetJS {t, v, f} cells), like Sheets' "Download → xlsx" — opening it in Excel shows the same formula, best-effort (TrueCalc's formula syntax is Sheets-flavored and is written through unadjusted, so an Excel-specific function could fail to recalculate there; the resolved value is unaffected either way).

A literal, a formula, and a format together are exactly what gets captured

Given

• A1 = 10

When I type =A1*2 into A2, then click A2, then press Control+b

Then

• A1 shows 10
• A2 shows 20
• A2 has formula =A1*2
• A2 is bold

Undo after formatting still leaves a value/formula round-trip-able

Given

• A1 = 5

When I type =A1+1 into B1, then click B1, then press Control+b, then undo

Then

• B1 shows 6
• B1 has formula =A1+1
• B1 is bold=false