Bags: Bag and Bank Columns sliders, and fix Window Scale not rescaling an open bank

[LiQiuDGG]

What does this PR do?

Two changes in the Bags module.

Bag Columns / Bank Columns sliders. Adds one options row exposing the grid width of the bag and bank windows. bagColumns already existed in the profile at 12 but was not reachable from the options page. bankColumns is new and defaults to 14, the value COLUMNS has always been hardcoded to in EllesmereUIBags/EllesmereUIBags_Bank.lua. Both windows therefore look exactly as they did until a slider is moved. The warband bank follows Bank Columns. bankColumns is added to the migration profile key list in EllesmereUI_Migration.lua so it carries across profiles and presets.

Window Scale on an open bank. The Window Scale setter guarded on _G.EUI_Bank, which is always nil: EUI_Bank is a file local, and the frame's global name is EUI_BankFrame (EllesmereUIBags/EllesmereUIBags_Bank.lua:209). The guard never passed, so dragging Window Scale while the bank was open did nothing. The bank already applies the same bagScale value on open (EllesmereUIBags/EllesmereUIBags_Bank.lua:2374), so only the live update was affected; closing and reopening the bank picked the new scale up. The fix uses the real global.

How was it tested?

Live on 12.1.0 (client 120100), running a package of this branch built from v9.0.0.

• Bags, bank and warband bank at default and raised column counts
• Window Scale dragged with the bank already open, before and after the fix
• /console scriptErrors 1 for the whole session, no errors
• Auto-Size to Fit on: Bag Columns behaves as the minimum its tooltip describes, with Auto-Size adding columns past it and never dropping below it
• Settings survive /reload
• bankColumns carries across a profile switch
• Bank Columns changed while the bank is closed applies on the next open

Screenshots

Before, on stock v9.0.0:

1. Bank open, Window Scale 100.

2. Window Scale 130. The inventory rescales; the bank, already open, does not.

After, on this branch. Each step changes one thing:

3. Defaults, Bag Columns 12 and Bank Columns 14.

4. Bag Columns 16 and Bank Columns 18, Window Scale unchanged.

5. Window Scale 125, columns unchanged. The bank now rescales.

6. The new options row at its defaults.

Checklist

• New settings default OFF (no behavior change without opt-in) — not applicable as written, these are sliders rather than toggles. They default to the values already in use, 12 for bags and 14 for the bank, so nothing changes for anyone who does not move them.
• Zero cost while disabled: no events registered, no polling, no hooks doing work, no frames built — none of those are added. The bank reads bankColumns inside its existing refresh path.
• Cheap while enabled: event-driven (no polling, no timer-based logic, no per-frame allocations) — one upvalue assignment per bank refresh.
• No writes onto Blizzard-owned frames (weak-table pattern used); HookScript/hooksecurefunc only, never SetScript on Blizzard frames — not applicable, only EllesmereUI's own frames are touched.
• Tested in-game on live; no version gates or pre-Midnight APIs added.