Restore the last-visited channel on startup

[Frodotus]

What

On startup, the active workspace now opens the most-recently-visited channel instead of always snapping to the sidebar's first entry — so a restart lands you back where you left off.

How

The channel_visits table already persists a per-channel last_visited timestamp (and is loaded into WorkspaceContext.LastVisitedByChannel at connect). This wires that existing data into the startup path:

• mostRecentlyVisitedChannel(visits) picks the channel with the latest timestamp; it's passed on WorkspaceReadyMsg.LastChannelID.
• reduceWorkspaceReady (the InitialActive branch) opens LastChannelID when it's set and still present in the sidebar, otherwise falls back to Channels[0].

Only the startup path changed; the existing mid-session workspace-switch restore (lastChannelByTeam) is untouched.

Edge cases

• First run / no recorded visit → LastChannelID is empty → opens Channels[0] (unchanged behavior).
• Last channel archived/left → lookup finds no match → falls back to Channels[0].

Testing

go test ./... passes; added TestWorkspaceReadyRestoresLastChannel covering restore, the empty-visit fallback, and the stale-channel fallback.

[gammons]

One trivial fix: cmd/slk/main.go is missing a blank line between the new mostRecentlyVisitedChannel function's closing brace and the // rtmEventHandler doc comment, which is a gofmt violation that CI/go fmt will flag — otherwise the change looks correct and well-tested.

[Frodotus]

Fixed in 03980a4 — added the blank line between mostRecentlyVisitedChannel and the rtmEventHandler doc comment. gofmt -l is clean on that change now.

[gammons]

🍔