Add QR code export: GUI on-screen display + CLI ASCII-art (#45)

[sidick]

Summary

QR import (decode) shipped in v1; this closes #45 by adding the reverse direction so an account can be added to a phone authenticator too.

• Core: otpauth_build() (URI builder, the inverse of otpauth_parse) and base32_encode() — neither existed yet, both needed before any QR rendering was possible.
• Vendored qrcodegen (Project Nayuki, MIT) alongside the existing quirc decoder — single .c/.h pair, no malloc, no floating point, pinned to a specific upstream commit (see THIRDPARTY.md).
• GUI: "Show QR code..." (Account menu, RAmiga+S) opens a window rendering the module grid via direct RastPort drawing, mirroring the existing led_draw() pattern.
• CLI: AmiAuth QR <account> prints ASCII-block art; forwards to a resident GUI via a new AAP_QR IPC command (same pattern as GET).
• Docs: Managing-Accounts.md, CLI-Reference.md, GUI-Guide.md, THIRDPARTY.md, ARCHITECTURE.md.

Bug found and fixed during on-target verification

qrcodegen_Mask_AUTO (searches all 8 masks + penalty-scores each) took over a minute on real m68k hardware for a modest QR code — confirmed via Copperline. Switched to a fixed mask and disabled boostEcl, cutting this to ~4 seconds with no loss of correctness — any valid mask produces a scannable code, AUTO only optimises which one scans best.

Test plan

• make test — 357 host tests pass, including a full encode → rasterize → decode round-trip through the real quirc decoder
• make m68k-docker / make gui-docker — plain-68000 cross-builds succeed, zero warnings
• Real on-target verification via Copperline: screenshot shows a correctly-rendered QR code (three finder patterns visible) in the GUI window after RAmiga+S
• AmiAuth QR <account> manually verified on host CLI, producing a visually correct QR pattern
• make guide (AmigaGuide doc conversion) succeeds

🤖 Generated with Claude Code