Extract hardcoded Chinese strings into res/values/strings.xml

[whtis]

Why this is useful

Many Compose screens use literal Chinese strings ("持仓", "加仓", "减仓", "自选", etc.). Moving them to res/values/strings.xml (zh-rCN default) and res/values-en/strings.xml (English) makes the app usable for non-Chinese speakers and is a prerequisite for any future locale support.

Where to start

Pick one screen at a time. A good first target:

• android/app/src/main/java/com/tis/ibkr/ui/screens/PositionsScreen.kt
• android/app/src/main/java/com/tis/ibkr/ui/screens/SettingsScreen.kt
• android/app/src/main/java/com/tis/ibkr/ui/nav/Tab.kt (tab labels)

Replace Text("持仓") with Text(stringResource(R.string.tab_positions)). Add the key to both values/strings.xml (Chinese) and values-en/strings.xml (English).

Definition of done

• At least the Positions tab, Settings tab, and bottom navigation labels are externalized.
• Switching device locale to English renders those screens without any Chinese.
• No regressions in zh-rCN.