fix(webui): persist Terminal/Chat/Program view selection per session

[edwin-zvs]

Problem

In the web UI, the view-mode selector (Terminal / Chat / Program) didn't fully follow the focused session. Terminal and Chat were already per-session (stored in viewModeById), but Program was treated as a global mode — it lived only in the global state.mode and was never written to viewModeById (both the load and save filters dropped "program").

Two concrete symptoms:

1. Program stuck globally. Switching sessions while viewing a Program carried Program onto the next session — selectSession keyed off state.mode === "program" (the outgoing session's mode) instead of the incoming session's own preference — so a terminal/chat session would render as Program just because the previous session was in Program.
2. Program not restored. A session you deliberately left in Program reopened in Terminal/Chat, because its Program choice was never stored or restored.

Fix

Make Program a real per-session preference, peer to Terminal and Chat — which is exactly what spec 0059 already declares it to be ("a third per-session view mode, peer to Terminal and Chat"). The implementation simply wasn't honoring it.

• Include "program" in the loadViewModePrefs / saveViewModePrefs filters for viewModeById.
• Store "program" against the current session when entering Program mode; drop a stale "program" entry when a single-surface session leaves Program (so it reverts to its natural surface).
• preferredViewModeForSession returns "program" when stored — it wins for any session, since every session owns a Program.
• selectSession restores the incoming session's preferred view, not the outgoing session's global state.mode.
• renderViewModeToggle highlights the per-session preference (not state.mode), so the toggle is correct during a switch, before the new surface is entered.

The session-deleted handler already cleaned up viewModeById, so removed sessions still drop their stored preference.

Behavior after

Each session independently remembers Terminal / Chat / Program and reopens in that view when you switch back to it. New sessions still default to their natural surface (Terminal for PTY-backed, otherwise Chat).

Spec

Adds specs/0062-webui-view-mode-is-per-session.md recording the per-session view-mode persistence/restore semantic so it isn't regressed back to a global toggle.

Testing

• cargo build (debug) green in the worktree. index.html is include_str!'d into the daemon, so it ships in the construct binary (construct daemon run).
• Inline-script parse check on index.html passes.

Only touches crates/daemon/assets/index.html (+ a new spec) → relevant binary is construct.