… locale files

The fila_manager web page was hardcoded in Chinese (lang="zh") with no
internationalization support. This commit adds a lightweight i18n system:

- Add locales/en.json and locales/zh_CN.json with all UI strings
- Add t(key) / tf(key, n) translation helpers loaded at startup
- Apply translations via data-i18n / data-i18n-placeholder attributes in HTML
- Replace all hardcoded Chinese strings in index.js with t() calls
- Language is resolved from the ?lang= URL parameter passed by C++

Previously untranslated strings added, covering:

Texture import workflow (new in v2.7):
- Multi-Color, Color Count, Smooth Level, Filament Mapping, New Filaments
- Skip Matching, Add Material, Import Model, Processing
- Computing/Applying texture colors, Texture Import Warning
- Auto-merge same filament, Reminder: parameters changed, etc.

Device status / calibration messages:
- Paused (filament ran out), Heating nozzle, Pause (nozzle clog/clumping)
- Pause (step loss/cutter error/front cover/Gcode/AMS offline)
- Pause (nozzle/heatbed temperature malfunction, chamber control problem)
- Calibrating flow ratio, dynamic flow, BirdsEye Camera
- Waiting for heatbed, Updating 3D view, Placing model on bed

Settings and descriptions:
- Purge Mode Fast/Standard, Flush multiplier, Override overhang speed
- Printer/Process/Filament presets (%d/%d), Auto Check Material
- Bed/Y-axis mass and force limits, Software Update, Current Version

UI labels and dialogs:
- Filter, Recommend, Old version, Operation, High Temp, Saving
- Nothing selected, Filament Manager, Skin/Skeleton infill pattern
- New Beta Available, Don't show Beta updates, Separate config folder
- File chooser dialogs, error messages, TPU warning

Fills previously missing msgstr entries in fr, es, it, nl, pl, ru, uk,
tr, pt_BR, sv, cs, hu, ko, ja covering the same string categories
already translated for de in a prior commit:

- Texture import workflow (Multi-Color, Color Count, Filament Mapping,
  Skip Matching, Computing/Applying texture colors, Texture Import
  Warning, Auto-merge same filament, etc.)
- Device status / pause messages (filament ran out, nozzle clog/clumping,
  step loss, AMS offline, heatbed/nozzle temperature malfunction,
  chamber control problem, Gcode by user, front cover, heatbreak fan)
- Calibration states (flow ratio, dynamic flow, BirdsEye Camera,
  Live View Camera, motion accuracy)
- General UI labels (Filter, Saving, Limits, Cool, Fast, Text,
  Software Update, Current Version, Filament Manager, Nothing selected,
  Purge Mode Fast/Standard, Printer/Process/Filament presets, etc.)
- Beta update flow (New Beta Available, Don't show again, Why use Beta,
  Try Now, Importing model, Updating 3D view, Model imported)