ux: show TUI chrome + a "loading…" overlay during binary open/analysis

Two parts:

1) A LoadingScreen modal is pushed on mount and dismissed once we land on a
   function (or the symbol picker). Its note line mirrors _status, so you see
   "opening… analyzing", then "N functions…", instead of staring at empty panes.
   Esc hides it if you'd rather watch the status bar.

2) The real fix for the dead air BEFORE the TUI drew anything: the launcher used
   to block on idb_open (full analysis of a big binary) *before* starting the
   TUI, so the overlay only flashed after the wait was already over. Now the
   launcher only adopts an already-open session (instant); otherwise it defers
   the open to the TUI via open_path, and _connect does idb_open (with the
   crashed-worker lock-sweep + retry moved here) while the chrome + overlay are
   on screen. Added a ttl arg to IdaTui for the deferred open.

Verified: overlay pushed on mount, present through open+function-load, dismissed
on landing; TUI-driven open (open_path, db=None) opens echo and lands on main
with notes "opening echo — analyzing…" -> "echo — 128 functions…"; startup/
palette/view_toggle/disasm_nav/search/rename/follow_xrefs/decomp_nav/mouse/
listing_view/continuous_view/func_banners green.

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