Fix: restore default white background fallback in Button:init (app-wide crash)

[jperon]

The problem

button.lua does:

local Button = require("ui/widget/button")
function Button:init() ... end

In Lua, require caches modules and always returns the same table. So this redefinition doesn't create a subclass local to kochess: it replaces the init method of the Button widget shared by the whole of KOReader. Once the chess board has been opened once in a session, every button in the application (menus, dialogs, etc.) uses this patched version, even outside the plugin.

This copy of the original code dropped the fallback to a default white background:

-- kochess's current version (button.lua)
self.frame = FrameContainer:new{
    ...
    background = self.background,  -- stays nil when no custom background is provided
    radius = self.radius,
    ...
}

whereas the original Button:init (frontend/ui/widget/button.lua) computes:

local background_color, border_color, radius
if self.background then -- colored button
    background_color = self.background
    border_color = background_color
    radius = self.radius or Size.radius.button
else
    background_color = Blitbuffer.COLOR_WHITE
    radius = self.radius
end

As a result, any "default" button (without a custom background) created anywhere else in KOReader after the chess board has been opened ends up with self.frame.background == nil. On the first tap on such a button, the highlight-feedback animation (flash_ui, enabled by default) crashes the whole app:

frontend/ui/widget/button.lua:406: attempt to index field 'background' (a nil value)

(Button:_doFeedbackHighlight calls self[1].background:invert() without checking that background isn't nil.)

Observed reproduction

On an Android e-reader (Bigme B6), after opening a chess game, the app consistently crashed when confirming any dialog (e.g. "Set as default" in the reader's configuration menu), without leaving a usable crash.log, since this is an uncaught Lua error rather than a native crash.

The fix

This PR restores the original fallback logic for background_color/border_color/radius, so that kochess's Button:init stays functionally equivalent to KOReader's for buttons without a custom background, while keeping kochess's own specifics (e.g. alpha support on IconWidget).

Longer term, it would probably be better for kochess to avoid monkey-patching the shared ui/widget/button module altogether (e.g. by extending Button via Button:extend{} instead of reassigning Button.init), so that any future divergence from upstream KOReader doesn't reintroduce this kind of app-wide regression. I haven't touched that architecture here, to keep the fix minimal and focused on the observed bug.