fix(linux): resolve default model and Electron sandbox crash on Ubuntu

[antmikinka]

Summary

• Wrong default model: On Ubuntu, Lemonade Server marks gpt-oss-mxfp4 as suggested: true, causing it to be auto-selected. Fixed by adding PREFERRED_MODEL_IDS priority in Preparing.tsx so Qwen3-Coder-30B-A3B-Instruct-GGUF is always selected first. Also updated the persisted defaults in SettingsRepository and StorageManager and bumped maxTokens from 2000 → 8192 to match Qwen3's capabilities.
• Electron sandbox SIGTRAP crash: On Linux, chrome-sandbox requires SUID root permissions that aren't present in typical dev environments. Fixed by disabling the sandbox via app.commandLine.appendSwitch('no-sandbox') in main.ts, gated to process.platform === 'linux' only. Windows/macOS are unaffected.
• Added npm run dev:linux script for Ubuntu development workflow.

Files Changed

File	Change
src/database/repositories/SettingsRepository.ts	Default model → Qwen3-Coder, maxTokens → 8192
src/database/storage/StorageManager.ts	Same
src/services/LemonadeClient.ts	Qwen3-Coder prepended to fallback model list
src/ui/pages/Preparing.tsx	PREFERRED_MODEL_IDS overrides server suggested flag
src/electron_app/main.ts	Linux sandbox disabled at process level
package.json	Added dev:linux scripts, structured build.linux target

Test Plan

• On Ubuntu: npm run dev:linux starts without SIGTRAP/sandbox crash
• On Ubuntu: Preparing page auto-selects Qwen3-Coder-30B-A3B-Instruct-GGUF instead of gpt-oss-mxfp4
• On Windows: npm run dev still works normally (sandbox unchanged)
• Fresh install (no existing settings.json): default model is Qwen3-Coder
• AppImage build succeeds: npm run build && npm run build:electron

Notes

• Existing Linux users with a persisted settings.json will not get the new default automatically (defaults only apply on first run), but the Preparing.tsx auto-selection fix will still pick Qwen3-Coder from the live Lemonade catalog regardless.
• The sandbox is disabled on Linux only. contextIsolation: true and nodeIntegration: false remain in place as the primary security boundary.

Closes #48

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