fix: ConPTY stdio leak, profile font CSS, xterm cell seams

[martin-ottosen]

What changed

• ConPTY stdio leak: detach the WPF app from any inherited std handles / console at startup. When launched from a parent that gives us redirected handles (e.g. dotnet run, bash background tasks), ConPTY children were inheriting those handles and writing to the launching shell instead of the PTY pipe — leaving xterm with no output and broken input.
• Profile font face quoting: Windows Terminal stores font faces as bare names like 0xProto Nerd Font. xterm.js drops the value straight into CSS font-family, where unquoted names with spaces fail to resolve. Quote the value (and append , monospace) both at import time and when applying overrides, so existing saved sessions render correctly without re-import.
• Cell seams on block characters: enable xterm's customGlyphs so box-drawing/block characters render with built-in glyphs that tile flush across cells.

Testing

• Launch the app from dotnet run (or any console parent). Create a session — terminal should print and accept input.
• Open a session that uses a Windows Terminal profile with a multi-word font face. Confirm the configured font is rendering, not the default Cascadia.
• Run something that draws box characters (e.g. tree, claude's welcome card). Confirm no thin seams between adjacent solid blocks.
• Existing sessions in state.json (with unquoted ProfileFontFamily) should pick up the correct font without manual re-import.

Developer notes

• The stdio detach uses FreeConsole + SetStdHandle(..., IntPtr.Zero). FreeConsole alone isn't enough — when the parent hands us pipe handles (not a console), only clearing the std handle slots prevents inheritance.
• customGlyphs only covers the box-drawing/block Unicode ranges. Nerd Font Private Use Area glyphs still seam in the DOM renderer; the proper fix for those is a canvas/webgl renderer addon, which would require pulling in a new asset. Not done here.
• Font face normalization is applied both at import (WindowsTerminalProfileService.QuoteFontFace) and at apply time (TerminalBridge.QuoteFontFamily) so saved sessions don't need to be re-imported to benefit.