Adds English/Chinese localization infrastructure with en-US as default, zh-CN as optional language, safe fallback behavior, settings integration, localized notifications, and expanded test coverage.

## Summary

Fixes the incomplete Simplified Chinese localization shipped in v1.3.0 and prepares v1.3.1.

## Changes

- Completed zh-CN localization coverage across the main UI.
- Added/updated localization keys for primary views, tray/status text, notifications, and settings.
- Kept en-US as the default language.
- Updated version metadata to 1.3.1.
- Changed installer display name to ThreadPilot while keeping version metadata separate.
- Added guarded cleanup for obsolete ThreadPilot 0.1.0-beta uninstall entry.
- Improved uninstall cleanup for ThreadPilot AppData, startup task, and HKCU Run entry.
- Added localization and packaging metadata tests.

## Validation

- dotnet build ThreadPilot_1.sln: passed, 0 warnings
- dotnet test ThreadPilot_1.sln: passed, 524 tests

## Safety

- No change to elevation behavior.
- No change to affinity/process control behavior.
- No change to power plan or system tweak behavior.
- Normal install/update preserves user settings.
- Full uninstall removes ThreadPilot-owned AppData for the uninstalling user.

## Summary

Fixes the release workflow smoke-test hang by handling `--smoke-test` early during app startup.

## Changes

- Detects `--smoke-test` before elevation, mutex, WPF UI startup, tray, or background services.
- Performs lightweight startup/package validation only.
- Exits explicitly with code 0 on success, 1 on validation failure, 2 on timeout.
- Adds regression tests for early smoke-test behavior.

## Validation

- `dotnet build ThreadPilot_1.sln`: passed
- `dotnet test ThreadPilot_1.sln`: passed, 527 tests

## Safety

- No normal startup behavior changes.
- No localization changes.
- No installer/uninstaller changes.
- No elevation/process/power plan behavior changes.

## Summary

Adds a safe in-app updater for ThreadPilot. This PR does not bump the app version and does not prepare a release; version and release metadata will be handled separately for v1.4.0.

## Type of Change

- [x] Feature
- [x] Security hardening
- [x] Documentation
- [x] Added/updated tests

## Update flow

- Checks for updates on demand from Settings.
- Performs background startup checks only when enabled and the last check is older than the configured interval.
- Defaults to a 7-day interval.
- Excludes prereleases by default.
- Notifies users when an update is available.
- Requires explicit user action before download/install.

## Security and safety

- Uses release metadata from the official `PrimeBuild-pc/ThreadPilot` repository only.
- Selects safe ThreadPilot installer assets from GitHub HTTPS release assets.
- Downloads into a ThreadPilot-owned temp update directory.
- Verifies SHA256 checksums when `SHA256SUMS.txt` is available.
- Rejects checksum mismatches.
- Performs best-effort Authenticode verification and rejects explicitly invalid signatures.
- Starts the installer elevated without shell command construction.
- Prevents concurrent install attempts.

## User data preservation

- Update flow does not delete AppData, settings, profiles, CPU masks, rules, custom/imported power plans, or logs.
- Only updater temp files are cleaned.
- Full uninstall behavior remains separate.

## Validation

- `dotnet restore ThreadPilot_1.sln`: passed
- `dotnet build ThreadPilot_1.sln`: passed, 0 warnings, 0 errors
- `dotnet test ThreadPilot_1.sln`: passed, 537 tests

## Release note

This PR intentionally leaves `ThreadPilot.csproj` at `1.3.1`. The `1.4.0` version bump, changelog, packaging metadata, and tag should happen in a separate release-prep branch after this PR is merged.

## Summary

Prepares the v1.4.0 release after the safe in-app updater feature merge.

## Changes

- Bumps project/package/installer metadata from 1.3.1 to 1.4.0.
- Updates changelog and release notes.
- Adds v1.4.0 release notes.
- Updates packaging examples where needed.

## Release highlights

- Safe in-app updater.
- Manual update checks from Settings.
- Optional background update checks with a default 7-day interval.
- Explicit user confirmation before installing updates.
- SHA256 verification when release checksums are available.
- Best-effort Authenticode verification.
- User data preservation during update.

## Validation

- `dotnet build ThreadPilot_1.sln`: passed
- `dotnet test ThreadPilot_1.sln`: passed