Implement Shift+Arrow cell selection resizing for sheet UI

[Hardyx07]

Fixes #4906

This PR adds the ability to expand or shrink your cell selection using Shift + Arrow keys in the sheet UI, as requested in #4906.

• Current behavior: Arrow keys move the active cell, but do not expand selection.
• New behavior: Holding Shift and pressing an Arrow key now expands (or contracts) the selection in the direction of the pressed key, starting from the active cell. This matches the Google Sheets selection workflow and greatly improves spreadsheet usability.

Technical details

• Updated cell selection logic to track selection origin and expand/collapse selection as Shift+Arrow keys are pressed.
• No breaking changes. Code follows established project style.
• Only relevant frontend files are changed. No unrelated code modifications.

Testing instructions

• Open any sheet
• Select any cell
• Hold Shift and press Arrow keys (up, down, left, right)
• Watch the selection expand or contract in the correct direction

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the develop branch of the repository
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.

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[Anish9901]

Hey @Hardyx07, can you please update the PR description?

[Hardyx07]

Hey @Hardyx07, can you please update the PR description?

Hi @Anish9901 , I’ve updated the PR description with more details as requested. Please let me know if anything else is needed!

[seancolsen]

There seem to be many inconsistencies between the behavior of Google Sheets and the behavior of this PR.

For example say you have this selection

Shift+Down works. Okay.

But the following keyboard shortcuts do not work the same way that Google Sheets works:

• Shift+Up
• Shift+Right
• Shift+Left

As I play with different shapes of selections it's very easy to find lots of different inconsistencies. I won't spend time trying to list them all out. This needs a lot more QA.

Also, looking at the code. Wow, that's much more code than I would expect this implementation to be. I've not spent time reading through it at all (because clearly some logic needs to be fixed first). But so many levels deep of if/else statements is a major code smell here. It's hard to read code like that and reason about it. My hunch is that you need to rethink the approach.

[Hardyx07]

@seancolsen Got it. I’ll rethink the approach and clean up the architecture.