The session-tree selector collapsed to "No entries found" whenever the
default filter rejected every entry. On a fresh session that is the
normal case: `sdk.ts` writes `model_change` + `thinking_level_change`
at startup so model + thinking state survive resumes, and the default
filter treats both as bookkeeping. `selector-controller.showTreeSelector`
guards on `tree.length === 0` so the selector still opens, and the user
sees an unexplained empty panel with `(0/0)` next to a "Recent sessions"
list that does contain data.
Split the empty-state branch into three shapes:
- `flatNodes.length === 0` → unchanged "No entries found".
- `searchQuery` non-empty → "No entries match search \"…\"" plus a
Backspace hint, with the real total in `(0/N)`.
- otherwise → "N entries hidden by the current filter [mode]" plus
"Press Alt+A to show all, Alt+D for default", with the real total in
`(0/N)`.
So the fresh-session case now explains why the panel is empty and how
to widen it instead of reading as "/tree is broken".
Fixes#1909
roboomp
ccc3533c45