fix(dashboard): align submenu rows with the rest of the menu

[suleimansh]

The bug

In the session ⋮ menu, Open in editor sat noticeably right of every other row.

DropdownMenuSubTrigger put justify-between on the row so the chevron would sit at the far end. But justifying the row spreads every child, not just the last one, so a trigger whose label is plain text had that label pushed toward the middle while ordinary items sat left against their icon.

The only way to sit straight was to remember a flex-1 wrapper around the label. AgentModelMenu and OptionsMenu happen to do that; SessionActionsMenu does not, in two places.

The fix

The chevron moves itself to the end with ml-auto, and the row keeps ordinary item layout. Both ways of writing a trigger now align identically, so the next call site cannot get it wrong.

Measured, not eyeballed

Driven in real Chrome against the live dashboard, reading the left edge of each label's glyphs with a DOM Range (an element box would measure the row for a bare-text label and the wrapper for a wrapped one, which is not a comparison).

Before:

Open on GitHub          723
Open project folder     723
Open in editor          775.8   <-
Copy resume command     723
Serve                   723
Delete session          723

After: all six at 723.

The second capture landed on a multi-package project, which also exercises the Serve trigger's submenu form. It aligns too.

Tests

dashboard 495 pass / 0 fail, typecheck and build clean. No unit test: this is a layout property jsdom cannot observe, so the browser measurement above is the evidence.