fix: make disabled buttons visible in light mode

[samdaw13]

Summary

• Adds disabledSurface and disabledText colour tokens to ColorPalette with theme-appropriate values for both light and dark modes
• Overrides React Native Paper's surfaceDisabled/onSurfaceDisabled theme tokens on the Run Analysis button (Break Cipher) and the Paste confirm button (Keyboard) so disabled state is legible in light mode

Test plan

• Switch to light mode and open the Break Cipher screen — verify the Run Analysis button is visible when disabled (before entering ciphertext)
• Open the paste modal on the Keyboard screen — verify the confirm button is visible when disabled (before entering text)
• Confirm both buttons still appear correctly in dark mode
• Confirm both buttons look correct when enabled in both themes

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