* fix(dashboard): only open the chat PTY once the chat tab is active
The dashboard mounts ChatPage persistently (hidden with CSS) on every route
so the embedded chat PTY survives tab switches. But the PTY-connect effect
never checked whether the chat tab was active, so it opened `/api/pty` on
mount for ANY dashboard page. On a source/RPi install that spawns the whole
TUI + agent bootstrap (`Installing TUI dependencies…` → `npm install`) merely
by loading /sessions, /system, etc. — work the user never asked for, and the
trigger behind "dashboard loses custom themes on /chat load".
Gate the connect effect on a sticky activation latch: the PTY is not spawned
until the chat tab has been active at least once, and stays connected across
later tab switches so the persistence UX is preserved.
* test(dashboard): cover chat PTY activation latch
Asserts the invariant behind the fix: activation is sticky. It stays false
while the chat tab has never been active (so the persistently-mounted,
hidden ChatPage never opens /api/pty), flips true when the tab activates,
and stays true after the user navigates away (PTY persistence).
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