feat: onboarding 2-keys + Windows install w/o admin (v0.7.3)

Two user-reported pain points:

1. First-run setup proposed only MiniMax (no MiMo step). Onboarding
   now offers both keys side-by-side under a single "apikey" step,
   with per-key Validate buttons. At least one must be valid to
   proceed; the rest of the providers (OpenAI/Anthropic/Z.AI/Ollama)
   are not shown in the wizard — they're configured later via the
   Config tab. Active provider = MiniMax if valid, else MiMo.

2. Windows install instructions failed: Move-Item to C:\Windows
   requires admin. Replaced with a no-admin 4-line snippet that
   installs to %LOCALAPPDATA%\Muyue and calls a new subcommand
   `muyue install-shortcuts` to create Desktop + Start Menu .lnk
   files and add the install dir to the user PATH. Shortcut creation
   uses WScript.Shell COM via PowerShell — keeps Go binary
   dependency-free. Folder paths resolved through
   [Environment]::GetFolderPath so OneDrive/redirected profiles
   work too.

- cmd/muyue/commands/install_shortcuts.go: new file
- web/src/components/OnboardingWizard.jsx: 2-key apikey step
- .gitea/workflows/ci-main.yml: updated install snippet
- internal/version/version.go: 0.7.2 → 0.7.3
- CHANGELOG.md: v0.7.3 entry

.gitea/workflows/ci-main.yml

@@ -138,11 +138,12 @@ jobs:
             echo "sudo mv muyue-darwin-arm64 /usr/local/bin/muyue"
             echo "\`\`\`"
             echo ""
-            echo "**Windows (x86_64)**"
+            echo "**Windows (x86_64)** — sans privilèges admin, crée les raccourcis Bureau + Menu Démarrer :"
             echo "\`\`\`powershell"
-            echo "Invoke-WebRequest -Uri \"${DL_URL}/muyue-windows-amd64.zip\" -OutFile \"muyue.zip\""
-            echo "Expand-Archive -Path \"muyue.zip\" -DestinationPath \".\""
-            echo "Move-Item muyue-windows-amd64.exe C:\\Windows\\muyue.exe"
+            echo "\$dest = \"\$env:LOCALAPPDATA\\Muyue\""
+            echo "Invoke-WebRequest -Uri \"${DL_URL}/muyue-windows-amd64.zip\" -OutFile \"\$env:TEMP\\muyue.zip\""
+            echo "Expand-Archive -Path \"\$env:TEMP\\muyue.zip\" -DestinationPath \$dest -Force"
+            echo "& \"\$dest\\muyue-windows-amd64.exe\" install-shortcuts"
             echo "\`\`\`"
           } > /tmp/stable_changelog.md
           echo "path=/tmp/stable_changelog.md" >> $GITHUB_OUTPUT