feat(sheets): add sheet-to-sheet hyperlinks

[the-narwhal]

Sheet-to-sheet hyperlinks

Implements the ability to create a clickable link in one sheet that navigates to another sheet within the same file — a frequently requested feature from the Proton community.

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⚠️ Testing note

I was unable to run or build the application locally (node_modules not installed in this environment), so this PR has not been manually tested end-to-end. It has been statically reviewed and audited for correctness, but it should go through your normal QA pass before merging.

This is intended as a reference implementation, feel free to merge it as-is, iterate on top of it, or simply use it as a starting point. All design decisions are documented below and open to feedback.

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What changed

sheetLink.ts (new)
A small utility module for encoding and decoding internal sheet links. Links are stored as the string #sheet:<sheetId>, using the numeric ID rather than the sheet name so that links survive renames. The parser uses a strict round-trip check (String(parseInt(raw)) === raw) to reject malformed values like #sheet:1abc or #sheet:1.5.

InsertLink.tsx
A segmented "Web link / Sheet in this file" toggle is added above the existing URL input. Selecting the sheet tab shows an Ariakit Select dropdown populated from sheets.list (visible sheets in tab order, matching the bottom bar). Opening the dialog on a cell that already carries a sheet link pre-selects the correct tab and sheet.

One correctness fix was required here: the original two-effect initialisation pattern (one to read the existing hyperlink, one to default to the first sheet) caused a React flush-time race where the defaulting branch overwrote the parsed sheet ID. These are now merged into a single effect.

CellTooltip.tsx
LinkInfo detects #sheet: links and handles them differently from external URLs:

	External URL	Sheet link
Icon	globe	grid-2
Display text	raw URL	sheet name (live, reflects renames)
Click	opens via event bus	showSheet() if hidden, then setActiveId()
Copy	copies URL	copies #sheet:<id> string
Copy disabled in read-only	yes (pre-existing)	no — copying is non-mutating

Sheet name resolution uses sheets.listIncludingHidden so that links pointing to a hidden sheet still display the correct name and remain clickable (clicking auto-unhides the sheet, matching the tab switcher behaviour in BottomBar). If the target sheet has been deleted the tooltip falls back to "Unknown sheet" and the click is a no-op.

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Design decisions open to feedback

• Link format #sheet:<id> is stored as a plain string through the existing onInsertLink API. This avoids any changes to the @rowsncolumns data layer but means the format lives entirely in our layer. An alternative would be to use the library's native { kind, location } object format if onInsertLink ever starts accepting it.

• No cell reference the current implementation links to a sheet only, not to a specific cell or range. Extending the format to #sheet:<id>!A1 and switching the click handler to goToCell() would be a natural follow-up.

• Hidden sheets in the picker the dropdown intentionally shows only visible sheets (sheets.list). If linking to a hidden sheet is a desired workflow, the picker could switch to sheets.listIncludingHidden with a visual indicator for hidden entries.