Add git identity config and zsh+starship shell with GUI theme, Add starship prompt config scaffolding and integration

[DimmKirr]

Changes

• feat(shell): add shell.nix with zsh, autosuggestions, syntax highlighting, and starship prompt configuration for consistent shell experience
• feat(desktop/themes/main): introduce new neobrutalist fluxbox theme with palette, fonts, pixmaps, wallpaper SVG, and fluxbox cfg generation
• chore(desktop): switch from old theme.nix to new main theme module; add kitty terminal with GPU-accelerated config matching theme colors
• chore(desktop): update fluxbox default.nix to use new theme and fonts; adjust fluxbox menu and init config with new paths and fonts
• fix(rdp): change xrdp SSL certificate directory from /etc/devcell/xrdp to /etc/devcell/config/xrdp to survive container restarts
• feat(internal/cfg): add [git] section with author/committer name and email; support identity fallback logic with explicit committer overrides
• feat(internal/runner): enhance docker run environment to inject Git identity env vars from host env, [git] toml config, or mount ~/.config/git/config, else use defaults
• refactor(cmd): change default shell in shell command and tests from bash to zsh to match new shell.nix profile
• test(container): add extensive e2e tests for git identity env vars, git config mounting, and commit metadata correctness
• test(container): add e2e shell tests verifying starship installation, config presence, prompt rendering, zsh integration, and plugin presence
• test(container): add desktop GUI tests verifying fluxbox theme deployment, wallpaper PNG validity, toolbar/menu/window colors and fonts, pixmap presence, Xresources loading, and active fluxbox session
• docs(README): add Shell section describing zsh and starship prompt; add Components section explaining nixhome modules composition and profiles
• chore(base): import shell.nix in base profile to enable new shell and prompt by default
• chore(internal/runner): mount global config dir /etc/devcell/config instead of xrdp path; modify related tests accordingly
• test(cmd): update shell related tests to expect zsh instead of bash as default shell
• internal/cfg: add tests for GitSection loading, merging, identity detection, and committer fallback behavior
• feat(scaffold): embed and scaffold homedir/.config/starship.toml for project prompt customization
• chore(entrypoint): copy starship.toml from project or default location into container homedir at startup
• chore(nixhome/desktop): add Mesa llvmpipe software rendering for GLX support and improve kitty terminal config
• test(shell): simplify zsh starship prompt integration test by sourcing .zshrc directly without PTY
• test(desktop): update GUI theme tests to reflect new toolbar height, fonts switched to JetBrainsMono Nerd Font, and window handle style
• test(desktop): improve xrdb loading test to retry for up to 5s due to deferred loading in entrypoint