[Feature Request] Provide a Portable (Green) Edition & Allow Custom Installation Directory on Windows

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Hi Jan Team,

First of all, thank you for developing such a great open-source, user-respecting AI workstation with native MCP support. It has been a pleasure using Jan as a local control console.

However, there is a major friction point on Windows regarding how Jan handles installation and default data paths:

Forced AppData Installation without Choice
The Windows installer (.exe) currently installs the app silently into %LOCALAPPDATA%\jan without allowing users to select a custom installation directory (e.g., D:\AI_Tools\Jan).

System Drive (C:) Bloat
Because local AI workflows generate massive amounts of data (GGUF models, caches, MCP logs, and runtime dependencies), hardcoding default paths into the C: drive creates severe storage bottlenecks for users running separate system SSDs and dedicated data drives.

Lack of a True Portable Edition
Currently, making Jan fully portable requires manual workarounds like creating Windows directory junction points (mklink).

Suggested Improvements:
Official Portable Version: Provide a .zip or Portable .exe release option that stores all app executables, configurations, and data relative to the executable root directory (./data).

Custom Destination Picker in Installer: Re-enable the standard NSIS directory selector step in the Windows installer so users can choose where Jan is installed.

Onboarding Path Selection: Prompt users during initial setup to choose their preferred Jan Data Folder before downloading large dependencies or initializing default paths.

Respecting user autonomy over disk management and directory structures would make Jan a much cleaner and truly green workstation for power users.

Thank you for your hard work and consideration!