fix: let a user's own tmux work inside a persistent session, and size terminal text on its own (#159)

[kipavy]

Closes #159.

Why tmux looked broken

Persistent sessions (on by default) wrap the remote shell in tmux -L voltius new-session -A -s voltius_<uuid>. tmux exports $TMUX into that shell, and a tmux client given no -L/-S takes its socket from $TMUX. So inside a Voltius session:

• tmux ls listed Voltius's private sessions (voltius_35917e3f…) instead of the user's own;
• tmux attach -t <their session> refused with "sessions should be nested with care, unset $TMUX to force";
• and the wrapper owned C-b, so even a forced nest lost every prefix key.

The reporter's screenshots show exactly this: Termius, which does not wrap, lists their real noa / nb2 / gsuid / ai-shell sessions.

What changed

persistent_exec_command now:

• prepends unset TMUX TMUX_PANE (screen: unset STY WINDOW) to the pane command, so the shell inside starts with a clean multiplexer environment. Nothing in Voltius reads those back — every control path names the socket and session explicitly;
• gives up the prefix with prefix None, written into the config file (cold server) and pushed with set -g (a server an earlier session left running, which never re-reads -f). Version-gated to tmux 2.1+, the first release that accepts None as a key name; older tmux keeps C-b but still gets the clean environment.

The wrapper stays invisible: status line off, no prefix, and the user's own tmux behaves as it does over plain ssh.

Terminal font size

The same report asks for a terminal font size independent of the overall UI scale. terminalFontSize existed only as a theme field, editable only in the Theme Creator, so changing it meant cloning a theme or scaling the whole interface.

• New Terminal Font Size card in Settings → Appearance, overriding the active theme. getActiveTheme folds the override in, so xterm, the CodeMirror editor, --t-terminal-font-size and the theme chip all agree.
• On mobile, a two-finger pinch on the terminal drives the same value.
• It is deliberately device-scoped — a size that reads well on a phone does not on a desktop — so it is not in the uiPreferences sync wire.
• The UI-scale card's stepper is now a shared StepperCard, used by both settings.

Verification

• cargo test --lib shell_integration: 20/20, including the dropbear payload-length cap.
• vitest run: 500 files / 3878 tests pass; tsc --noEmit clean.
• The generated wrapper was rendered and syntax-checked under both dash and bash, and the tmux version gate was table-tested (3.4, 3.5a, 2.1, 2.9a → prefix None; 2.0, 1.8, empty → left alone).

Live run

Both halves were driven in the headless dev build, not just compiled.

The wrapper was run against a real tmux 3.7c (installed on the throwaway ssh-host-1), with the payload rendered exactly as persistent_exec_command emits it. Inside the wrapper's pane:

TMUX=[] TMUX_PANE=[]
user tmux ls:      mine: 1 windows (created Mon Aug 24 15:00:47 2026)
voltius socket ls: voltius_probe: 1 windows (attached)
wrapper prefix:    prefix None

The user's own tmux ls now lists their session and not ours, persistence still works on the private socket, and tmux attach -t mine from inside the pane attached — status line drawn, tmux list-clients -t mine reports 1 client — where it used to refuse with "sessions should be nested with care".

In the app: Settings → Appearance shows the new card; stepping it 14 → 20px moved --t-terminal-font-size to 20px and persisted terminalFontSize: 20. With a live local shell open, 12px vs 26px visibly resizes the terminal text (grid refits to 67×21) while the settings modal, sidebar and tab bar stay pixel-identical. Reset returns to the theme's 14px and clears the override; the UI-scale card still steps 100% → 105% → reset after the shared-stepper refactor.

The mobile pinch is covered by unit tests only — no touch device was driven.