Fix i18n: invalid default, English fallback, absolute config path

[minh-vt]

Problem

The app has a complete i18n system (8 languages, full en.ini), but three bugs make it fragile:

1. P0 — Fresh install crash: Config.interface defaulted to "ChineseSimplified", which isn't in the validator's option set ['ch', 'chinese_cht', 'en', ...]. On first launch with no config.json, ComboBoxSettingCard throws KeyError.

2. P1 — Missing translation keys crash non-English users: Every UI string uses tr['Section']['Key'] — direct dict access. If any non-English .ini is missing a key, it's an unhandled KeyError for all users of that language.

3. P2 — Config path is CWD-dependent: qconfig.load('config/config.json', config) resolves relative to the current working directory. Run from anywhere except the project root and settings silently fail to load.

Changes in backend/config.py

Before	After	Fix
interface = OptionsConfigItem(..., "ChineseSimplified", ...)	interface = OptionsConfigItem(..., "ch", ...)	Default valid from first boot
tr.read(f"{user}.ini") — single file	Load en.ini as base, overlay user language	Missing keys fall back to English
CONFIG_FILE = "config/config.json"	Absolute path via os.path.abspath(__file__)	Works from any CWD
f"en.ini"	"en.ini"	Remove unnecessary f-string

Files changed: 1