feat(pilot): add remote desktop groups and reliable connection recovery

[joeblau]

Remote Desktop now organizes saved machines into renamable groups with a fixed 2×2 grid. Each group holds up to four machines; unused slots remain black, and moving or removing a machine preserves the other screen positions. Existing connections are assigned to groups automatically. Group names, selection, and positions persist locally, and all four visible sessions stay active together.

Connection attempts now time out after 20 seconds with an actionable error and Try Again. Cancel, failure, and replacement invalidate old callbacks and deadlines. Saved passwords survive cancel, timeout, and dropped connections and refill the retry form; only an explicit server credential rejection clears them (or opting out of password saving).

Validation:

• 27 focused group, session lifecycle, credential, and session manager tests passed, including real loopback handshake stalls and Keychain retention checks.
• Render checks passed at 1000×800 and 640×480; both layouts were visually inspected and empty cells verified black.
• Strict Swift lint passed for all changed files, and the pinned XcodeGen reproduces the committed project.