fix(web): pin "Add workspace…" below the New session workspace list

[ericleepi314]

Why

With a long list of workspaces, the "Create new workspace…" option at the end of the New session dialog's native select was hard to find. The reference shows the workspace list with Add workspace… pinned below it in its own section.

What changed

• The dialog's workspace picker is now the app's own Menu: a row per folder with a folder glyph, the current one checked, the full path as a second line only where two folders share a name (so two clawcodex checkouts stay distinguishable while the common case stays one compact line). The list scrolls; Add workspace… sits below it, after a divider, pinned in view at any list length. Picking it reveals the folder-path field as before.
• Menu gains footer (rows rendered outside the scrolling viewport, after a divider, never marked selected) and block (the trigger fills the owner, the list spans it).
• Latent Menu bug fixed: rows/separators/labels are flex items of a height-constrained viewport with an explicit min-height, so a long list squashed every row to its minimum before scrolling — a two-line row's hint overlapped the next row. Unreachable before this change (the composer menus are short); flex: none on those rows.
• Escape and a press outside the open list close only the list; the next one closes the dialog. Focus returns to the trigger when the list closes, and the trigger's accessible name carries the chosen folder ("Workspace: /path").
• Menu measures the room above and below its anchor once open (and on resize), caps the list to the room on its side, and opens on the other side when its own has too little and the other has more (menuPlacement, unit-tested). Before, the stylesheet capped the list by the window's height regardless of where the anchor sat, so on a 1366×768 window the picker ran below the fold with the page unable to scroll — the pinned row was off-screen. The composer pickers keep opening upward.
• Cascade fix: the block list's width rule now compounds with .list and follows it, so the list spans its trigger (it was 360px under a 402px trigger).

Verification

• vitest: 692 tests green (new: pinned footer outside the viewport after a divider with 41 rows, selection check, add-workspace flow, Escape/outside-press scoping, Menu footer/block); tsc clean; bundle builds.
• Browser (served ui-web/dist on this machine's 37-workspace sidebar) at 1366×768, 1440×900 and 1280×640: the list spans the trigger (402px), scrolls, and its pinned Add workspace… row is within the viewport at every size (list capped to 366px / 420px / 302px); hinted rows 50px and plain rows 34px with no overlap; picking a row returns focus to the trigger; the composer's model picker still opens upward and in view; no console errors.
• One critic review round applied (viewport fit, cascade order, accessible name, focus return, Escape closure, name truncation).

🤖 Generated with Claude Code

[ericleepi314]

Deferred follow-ups from the review (non-gating, shared with the composer pickers built on the same Menu):

• Keyboard: the menu has no arrow-key navigation or typeahead, and Tab walks the rows and continues past the footer into the next control with the list still open. aria-haspopup="menu" promises the menu-button pattern; worth a Menu-primitive follow-up that would cover the model, effort and permission pickers too.
• Click-through: a press outside the open list but inside the dialog card closes the list and also activates the target (Create submits, the Worktree switch toggles). A native select swallowed that press.