Tabbed terminal panel, and drag-to-scroll a pane

[whaclaw-bot]

Two features for the browser viewer, plus the refactor that made room for them.

Tabs instead of a split. The panel can draw its panes one at a time behind a
tab strip; the button beside + switches between that and the side-by-side
grid. A narrow screen starts on tabs, because a split grid gives each pane fewer
columns than a command line needs. The choice is stored per device rather than on
the server — what a phone should do with four panes is not what the desktop
beside it should do.

Nothing about the session changes: + still sends create, tabs sit in pane
order, and a tab you are not looking at is a running program whose output keeps
arriving. Every pane is held at the panel's full size while tabbed, so switching
costs no resize — a zero-size cell would defer opening the terminal and skip its
sizing, making each switch a fit plus a SIGWINCH plus a full repaint.

A tabbed panel shows no zoom button (it already shows one pane), and a zoom
another client set does not move the keyboard there.

Drag a pane to scroll it. A finger dragged up or down turns the same wheel a
mouse would and lets xterm route it, so where it goes is up to the program in the
pane — a mouse-reporting agent gets its SGR report, less under alternate scroll
gets arrow keys, a plain shell scrolls the scrollback. defaultPrevented is the
seam; what xterm did not claim is scrolled with term.scrollLines, since a
synthetic wheel cannot drive the browser scrolling xterm's scrollback relies on.
A short drag is still a tap, so placing the cursor and pinching to zoom survive.

Verified in a browser at a phone viewport against a live daemon: SGR reports
reaching the PTY on a mouse-reporting pane, and a bash pane's scrollback moving
symmetrically both ways; + adding a tab rather than splitting, with the hidden
pane holding the same box as the visible one.

Gates: cargo build, cargo test (1498), cargo clippy -D warnings, tsc -b,
vitest run (205), and a viewer-ui/dist rebuilt from this source.