Simplify first-use account setup and browser icons

[squarepots]

First-time users can now sign in directly from the empty state. The empty state and Add account dialog share the same choices: a prominent Sign in action, Find on this computer, and Choose files, with auth JSON and API keys under Other methods. Browser sign-in and reauthentication actions use Lucide's tabbed-window icon; the Remix globe remains the language-menu icon.

Opening Find on this computer still requires an explicit Scan before reading local sources. The existing unsaved-current-account shortcut remains available. The change reuses the existing OAuth and import commands and updates the design and testing documentation. Version files remain at 1.0.22.

Validation:

• pnpm run ci:source frontend passed: 153 tests, version consistency, and production TypeScript/Vite build.
• A browser walkthrough with fictional accounts covered English and Simplified Chinese, light and dark themes, 1360 × 900 and 700 × 520 windows, direct sign-in and cancellation, selected local imports, file import, Other methods, keyboard dismissal, reauthentication, and Save current account.
• The maintainer accepted the clickable preview and requested this build and merge. Real-account/provider and installed-app behavior were not exercised.

Windows NSIS preview:

• Source: 3d9d1a6d1f0869dca466583fb90c62f13febba3a; preview version: 1.0.23-rc.1, supplied through an ignored Tauri configuration overlay.
• Maintainer-local installer: C:\Users\zhu13\Desktop\Github\GSwitch\src-tauri\target\first-use-preview\GSwitch_1.0.23-rc.1_x64-setup.exe (4,473,576 bytes).
• SHA256: dc91699e9b2f8c3ff89469dcf4741e2230f36f877206ba08bd8854338caf3214.
• The native release build and NSIS packaging passed. Installer and packaged EXE version metadata match 1.0.23-rc.1; the archive contains gswitch.exe and the uninstaller. Extraction passed, and the packaged EXE differs from the built EXE only by the three-byte bundle marker defined in the pinned Tauri source.
• This is an unsigned local preview. Interactive installation, real-account behavior, and public release publication were not performed.