Fix duplicate Alt+C access keys on Clear history/memory buttons (a11y)

[gebrilfradj]

Description

Accessibility Insights flagged a duplicate access key: three controls all declared Alt+C in en-US/Resources.resw:

• CategoryName_CurrencyAccessKey — Currency converter nav item
• ClearHistory.AccessKey — Clear all history
• ClearMemory.AccessKey — Clear all memory

Microsoft accessibility guidelines require access keys to be unique across the visual tree. This PR keeps C for Currency (the natural mnemonic) and assigns unique, mnemonic access keys to the two Clear buttons:

• ClearHistory.AccessKey: C → Y (clear historY)
• ClearMemory.AccessKey: C → R (cleaR memory)

Both replacement letters appear in the visible button label, so the underlined mnemonic renders sensibly, and neither collides with any other access key in the resource set or with any Alt-based keyboard accelerator (only Alt+Up/Alt+Down exist).

Only en-US/Resources.resw is changed; the other locale files are updated separately by the localized-strings pipeline, per existing repo convention.

How changes were validated

• Enumerated every *.AccessKey value in Resources.resw and every hardcoded AccessKey="..." in XAML, grouped by value: C now appears once (Currency), and Y/R each appear once — no duplicates introduced.
• Confirmed no C++/C#/test code depends on the previous literal values; existing UI tests invoke these buttons by AutomationId (ClearHistoryButton, PanelClearMemoryButton), unaffected by the access-key change.
• No automated test added: this is a static resource-string uniqueness change with no behavioral surface, consistent with recent resource-only fixes in this repo. Existing CI checks cover the build.