feat(sheets): remember last active sheet on reopen

[the-narwhal]

Note on Testing

This PR was implemented via static analysis of the codebase only. The editor runs inside an iframe with a full Proton Drive/RTS backend, so end-to-end verification wasn't possible in this context. The change is submitted as a complete, ready-to-review implementation for Proton to merge as-is, iterate on, or use as a reference. Unit-level logic is straightforward and self-contained; the main thing that warrants a real integration smoke test before shipping is confirming documentId is stable and populated by the time initializeEditor fires for the sheet case.

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Remember Last Active Sheet on Reopen

Closes the community-requested UX gap where opening a multi-worksheet spreadsheet always landed on tab 1, regardless of where the user left off.

Problem

handleInitialLoad in useYjsState unconditionally called onChangeActiveSheet(firstSheetId) after receiving the initial RTS sync. There was no persistence of the active sheet between sessions.

Solution

Two small additions to useYjsState in state.ts:

On open: restore saved sheet
handleInitialLoad now checks localStorage before defaulting to sheet 1. If a saved sheet ID is found and the sheet still exists in the document, it is activated instead. If the saved sheet has since been deleted, or this is the first time the document is opened, it falls back to sheet 1 exactly as before.

On tab switch: persist active sheet
A new saveActiveSheetToStorage effect writes the current activeSheetId to localStorage whenever it changes, gated on receivedEverythingFromRTS to avoid persisting the library's pre-load default value.

Storage key design
Keys are scoped to both documentId and userName so that collaborators on the same shared spreadsheet each independently remember their own last position:

proton-sheets-active-sheet:{documentId}:{userName}

Files Changed

• state.ts — all logic lives here: getActiveSheetStorageKey helper, documentId added to YjsStateDependencies (which automatically propagates to ProtonSheetsStateDependencies via the existing type spread), modified handleInitialLoad, new saveActiveSheetToStorage effect
• Spreadsheet.tsx — documentId: string added to SpreadsheetProps, threaded into useProtonSheetsState
• App.tsx — documentId={editorConfig.current.documentId} passed to <Spreadsheet>, mirroring the existing pattern already used for <Editor>

What Was Not Changed

• No changes to the Yjs document content or RTS sync; this is purely client-side localStorage
• No changes to sheet switching UX or collaborator cursor following
• Fallback to sheet 1 is fully preserved for first-open and deleted-sheet cases
• Read-only and revision modes are unaffected

Integration Smoke Test Recommended

Before merging, confirm:

1. Open a spreadsheet, switch to a non-first sheet, close and reopen. Should land on that sheet.
2. Open the same spreadsheet as a second user. Their position should be tracked independently.
3. Switch to a sheet, delete it, then reopen. Should fall back to sheet 1 without errors.
4. Open a brand new spreadsheet for the first time. Should open on sheet 1 as before.